497

SIDLEY AUSTIN LLP 787 SEVENTH AVENUE NEW YORK, NY 10019 +1 212 839 5300 +1 212 839 5599   AMERICA • ASIA PACIFIC • EUROPE

December 3, 2018

VIA EDGAR

Securities and Exchange Commission

100 F Street, NE

Washington, DC 20549

Re: BlackRock Summit Cash Reserves Fund, a series of BlackRock Financial Institutions Series Trust

(Securities Act File No. 002-78646, Investment Company Act No. 811-03189)

Ladies and Gentlemen:

On behalf of BlackRock Financial Institutions Series Trust, attached for filing are exhibits containing interactive data format risk/return summary information that mirrors the risk/return summary information in the Supplement to the Summary Prospectus and Prospectus for Investor A Shares of BlackRock Summit Cash Reserves Fund (the “Fund”), each dated July 27, 2018, as filed pursuant to Rule 497(e) under the Securities Act of 1933 on November 19, 2018 (the “497 Filing”). The purpose of this filing is to submit the 497 Filing in XBRL for the Fund.

Any questions or comments on the 497 Filing should be directed to the undersigned at (212) 839-8615.

Very truly yours,

/s/ Douglas E. McCormack

Douglas E. McCormack

Enclosures

cc: Benjamin Archibald, Esq.

BlackRock Advisors, LLC

Sidley Austin (NY) LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	497
Document Period End Date	dei_DocumentPeriodEndDate	Apr. 30, 2018
Registrant Name	dei_EntityRegistrantName	BLACKROCK FINANCIAL INSTITUTIONS SERIES TRUST
Central Index Key	dei_EntityCentralIndexKey	0000353281
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Nov. 19, 2018
Document Effective Date	dei_DocumentEffectiveDate	Nov. 19, 2018
Prospectus Date	rr_ProspectusDate	Jul. 27, 2018
Entity Inv Company Type	dei_EntityInvCompanyType	N-1A

XML

Jul. 27, 2018

BlackRock Financial Institutions Series Trust

BlackRock Summit Cash Reserves Fund

(the “Fund”)

Supplement dated November 19, 2018 to the

Investor A Shares

Prospectus and the Statement of Additional Information of the Fund,

each dated as of July 27, 2018, as supplemented to date

Effective as of November 19, 2018, BlackRock Advisors, LLC has contractually agreed to cap net expenses of Investor A Shares of the Fund. To achieve this expense cap, BlackRock has agreed to waive and/or reimburse fees or expenses if the annual fund operating expenses of Investor A Shares of the Fund, excluding certain expenses described in the Investor A Shares prospectus of the Fund (the “Prospectus”), exceed a certain limit. Accordingly, the Prospectus is amended as follows:

Effective as of November 19, 2018, the section of the Prospectus entitled “Fund Overview—Key Facts About BlackRock Summit Cash Reserves Fund—Fees and Expenses of the Fund” is deleted in its entirety and replaced with the following:

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. More information about these fees and expenses is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Advisors, LLC (“BlackRock”), The PNC Financial Services Group, Inc. and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 11 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-28 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	Investor A Shares
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)		None
Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)		None	1
Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Investor A Shares
Management Fee		0.50	%
Distribution (12b-1) Fees		None
Other Expenses		0.43	%
Total Annual Fund Operating Expenses		0.93	%
Fee Waivers and/or Expense Reimbursements2		(0.51	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2		0.42	%

¹  There is no contingent deferred sales charge (“CDSC”) on Investor A Shares purchased directly. However, a CDSC of up to 1.00% may apply to certain redemptions of the Fund’s Investor A Shares purchased in an exchange transaction for Investor A Shares of a non-money market fund advised by BlackRock or its affiliates (each a “Non-Money Market BlackRock Fund”) where no initial sales charge was paid at time of purchase of such Non-Money Market BlackRock Fund as part of an investment of $1,000,000 (lesser amounts may apply depending on the Non-Money Market BlackRock Fund) or more. The Investor A Shares CDSC is only charged upon redemptions of Investor A Shares within 18 months after you originally acquired such Investor A Shares of the Non-Money Market BlackRock Fund, unless you qualify for a waiver. There is no CDSC charged on redemptions if you have owned your Investor A Shares for more than 18 months (as measured from your original purchase of Investor A Shares that you exchanged into Investor A Shares of the Fund).

²  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 23, effective November 19, 2018, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.42% for Investor A Shares through August 31, 2021. The agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	43	$	192	$	412	$	1,047

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BLACKROCK FINANCIAL INSTITUTIONS SERIES TRUST

Prospectus Date

rr_ProspectusDate

Jul. 27, 2018

Supplement to Prospectus [Text Block]

rr_SupplementToProspectusTextBlock

BlackRock Financial Institutions Series Trust

BlackRock Summit Cash Reserves Fund

(the “Fund”)

Supplement dated November 19, 2018 to the

Investor A Shares

Prospectus and the Statement of Additional Information of the Fund,

each dated as of July 27, 2018, as supplemented to date

Effective as of November 19, 2018, BlackRock Advisors, LLC has contractually agreed to cap net expenses of Investor A Shares of the Fund. To achieve this expense cap, BlackRock has agreed to waive and/or reimburse fees or expenses if the annual fund operating expenses of Investor A Shares of the Fund, excluding certain expenses described in the Investor A Shares prospectus of the Fund (the “Prospectus”), exceed a certain limit. Accordingly, the Prospectus is amended as follows:

Effective as of November 19, 2018, the section of the Prospectus entitled “Fund Overview—Key Facts About BlackRock Summit Cash Reserves Fund—Fees and Expenses of the Fund” is deleted in its entirety and replaced with the following:

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. More information about these fees and expenses is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Advisors, LLC (“BlackRock”), The PNC Financial Services Group, Inc. and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 11 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-28 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	Investor A Shares
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)		None
Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)		None	1
Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Investor A Shares
Management Fee		0.50	%
Distribution (12b-1) Fees		None
Other Expenses		0.43	%
Total Annual Fund Operating Expenses		0.93	%
Fee Waivers and/or Expense Reimbursements2		(0.51	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2		0.42	%

¹  There is no contingent deferred sales charge (“CDSC”) on Investor A Shares purchased directly. However, a CDSC of up to 1.00% may apply to certain redemptions of the Fund’s Investor A Shares purchased in an exchange transaction for Investor A Shares of a non-money market fund advised by BlackRock or its affiliates (each a “Non-Money Market BlackRock Fund”) where no initial sales charge was paid at time of purchase of such Non-Money Market BlackRock Fund as part of an investment of $1,000,000 (lesser amounts may apply depending on the Non-Money Market BlackRock Fund) or more. The Investor A Shares CDSC is only charged upon redemptions of Investor A Shares within 18 months after you originally acquired such Investor A Shares of the Non-Money Market BlackRock Fund, unless you qualify for a waiver. There is no CDSC charged on redemptions if you have owned your Investor A Shares for more than 18 months (as measured from your original purchase of Investor A Shares that you exchanged into Investor A Shares of the Fund).

²  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 23, effective November 19, 2018, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.42% for Investor A Shares through August 31, 2021. The agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	43	$	192	$	412	$	1,047

BlackRock Summit Cash Reserves Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement to Prospectus [Text Block]

rr_SupplementToProspectusTextBlock

BlackRock Financial Institutions Series Trust

BlackRock Summit Cash Reserves Fund

(the “Fund”)

Supplement dated November 19, 2018 to the

Investor A Shares

Prospectus and the Statement of Additional Information of the Fund,

each dated as of July 27, 2018, as supplemented to date

Effective as of November 19, 2018, BlackRock Advisors, LLC has contractually agreed to cap net expenses of Investor A Shares of the Fund. To achieve this expense cap, BlackRock has agreed to waive and/or reimburse fees or expenses if the annual fund operating expenses of Investor A Shares of the Fund, excluding certain expenses described in the Investor A Shares prospectus of the Fund (the “Prospectus”), exceed a certain limit. Accordingly, the Prospectus is amended as follows:

Effective as of November 19, 2018, the section of the Prospectus entitled “Fund Overview—Key Facts About BlackRock Summit Cash Reserves Fund—Fees and Expenses of the Fund” is deleted in its entirety and replaced with the following:

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. More information about these fees and expenses is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Advisors, LLC (“BlackRock”), The PNC Financial Services Group, Inc. and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 11 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-28 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees (fees paid directly from your investment)	Investor A Shares
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)		None
Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)		None	1
Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Investor A Shares
Management Fee		0.50	%
Distribution (12b-1) Fees		None
Other Expenses		0.43	%
Total Annual Fund Operating Expenses		0.93	%
Fee Waivers and/or Expense Reimbursements2		(0.51	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2		0.42	%

¹  There is no contingent deferred sales charge (“CDSC”) on Investor A Shares purchased directly. However, a CDSC of up to 1.00% may apply to certain redemptions of the Fund’s Investor A Shares purchased in an exchange transaction for Investor A Shares of a non-money market fund advised by BlackRock or its affiliates (each a “Non-Money Market BlackRock Fund”) where no initial sales charge was paid at time of purchase of such Non-Money Market BlackRock Fund as part of an investment of $1,000,000 (lesser amounts may apply depending on the Non-Money Market BlackRock Fund) or more. The Investor A Shares CDSC is only charged upon redemptions of Investor A Shares within 18 months after you originally acquired such Investor A Shares of the Non-Money Market BlackRock Fund, unless you qualify for a waiver. There is no CDSC charged on redemptions if you have owned your Investor A Shares for more than 18 months (as measured from your original purchase of Investor A Shares that you exchanged into Investor A Shares of the Fund).

²  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 23, effective November 19, 2018, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.42% for Investor A Shares through August 31, 2021. The agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year		3 Years		5 Years		10 Years
Investor A Shares	$	43	$	192	$	412	$	1,047

Expense [Heading]

rr_ExpenseHeading

<b>Fees and Expenses of the Fund</b>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. More information about these fees and expenses is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Advisors, LLC (“BlackRock”), The PNC Financial Services Group, Inc. and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 11 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-28 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<b>Shareholder Fees<br/> (fees paid directly from your investment)</b>

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<b>Annual Fund Operating Expenses<br/>(expenses that you pay each year as a percentage of the value of your investment)</b>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

November 19, 2018<br/><br/>August 31, 2021

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

There is no contingent deferred sales charge (“CDSC”) on Investor A Shares purchased directly. However, a CDSC of up to 1.00% may apply to certain redemptions of the Fund’s Investor A Shares purchased in an exchange transaction for Investor A Shares of a non-money market fund advised by BlackRock or its affiliates (each a “Non-Money Market BlackRock Fund”) where no initial sales charge was paid at time of purchase of such Non-Money Market BlackRock Fund as part of an investment of $1,000,000 (lesser amounts may apply depending on the Non-Money Market BlackRock Fund) or more.

Expense Example [Heading]

rr_ExpenseExampleHeading

<b>Example: </b>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

BlackRock Summit Cash Reserves Fund | Investor A Shares

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOfferingPrice

none

^[1]

Management Fee

rr_ManagementFeesOverAssets

0.50%

Distribution (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.43%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.93%

Fee Waivers and/or Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.51%)

^[2]

Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

rr_NetExpensesOverAssets

0.42%

^[2]

1 Year

rr_ExpenseExampleYear01

$ 43

3 Years

rr_ExpenseExampleYear03

192

5 Years

rr_ExpenseExampleYear05

412

10 Years

rr_ExpenseExampleYear10

$ 1,047

[1] There is no contingent deferred sales charge (“CDSC”) on Investor A Shares purchased directly. However, a CDSC of up to 1.00% may apply to certain redemptions of the Fund’s Investor A Shares purchased in an exchange transaction for Investor A Shares of a non-money market fund advised by BlackRock or its affiliates (each a “Non-Money Market BlackRock Fund”) where no initial sales charge was paid at time of purchase of such Non-Money Market BlackRock Fund as part of an investment of $1,000,000 (lesser amounts may apply depending on the Non-Money Market BlackRock Fund) or more. The Investor A Shares CDSC is only charged upon redemptions of Investor A Shares within 18 months after you originally acquired such Investor A Shares of the Non-Money Market BlackRock Fund, unless you qualify for a waiver. There is no CDSC charged on redemptions if you have owned your Investor A Shares for more than 18 months (as measured from your original purchase of Investor A Shares that you exchanged into Investor A Shares of the Fund).

[2] As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 23, effective November 19, 2018, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.42% for Investor A Shares through August 31, 2021. The agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK FINANCIAL INSTITUTIONS SERIES TRUST
Prospectus Date	rr_ProspectusDate	Jul. 27, 2018
Document Creation Date	dei_DocumentCreationDate	Nov. 19, 2018