-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 TRXlw5hec/LwEw7vR8MKr5zxKQ18M7F7A+VMTsiXy5Ikp3qvuN+rrd6ZkBnkNhPB
 OLE9a4olUGdt/bklfOL8fg==

<SEC-DOCUMENT>0000950103-10-001095.txt : 20100414
<SEC-HEADER>0000950103-10-001095.hdr.sgml : 20100414
<ACCEPTANCE-DATETIME>20100414110446
ACCESSION NUMBER:		0000950103-10-001095
CONFORMED SUBMISSION TYPE:	D/A
PUBLIC DOCUMENT COUNT:		1
ITEM INFORMATION:		Rule 506
ITEM INFORMATION:		Investment Company Act Section 3(c)
ITEM INFORMATION:		Section 3(c)(7)
FILED AS OF DATE:		20100414
DATE AS OF CHANGE:		20100414
EFFECTIVENESS DATE:		20100414

FILER:

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			J.P. Morgan-BlackRock Opportunistic RMBS Master Fund L.P.
		CENTRAL INDEX KEY:			0001473669
		IRS NUMBER:				980629949
		STATE OF INCORPORATION:			E9

	FILING VALUES:
		FORM TYPE:		D/A
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	021-135431
		FILM NUMBER:		10748649

	BUSINESS ADDRESS:	
		STREET 1:		345 PARK AVENUE
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10154
		BUSINESS PHONE:		212-464-1205

	MAIL ADDRESS:	
		STREET 1:		345 PARK AVENUE
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10154

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	J.P. Morgan-TCW Opportunistic RMBS Master Fund L.P.
		DATE OF NAME CHANGE:	20091001
</SEC-HEADER>
<DOCUMENT>
<TYPE>D/A
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0704</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001473669</cik>
        <entityName>J.P. Morgan-BlackRock Opportunistic RMBS Master Fund L.P.</entityName>
        <issuerAddress>
            <street1>270 PARK AVENUE</street1>
            <city>NEW YORK</city>
            <stateOrCountry>NY</stateOrCountry>
            <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
            <zipCode>10017</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>(212) 464-1205</issuerPhoneNumber>
        <jurisdictionOfInc>CAYMAN ISLANDS</jurisdictionOfInc>
        <edgarPreviousNameList>
            <previousName>J.P. Morgan-TCW Opportunistic RMBS Master Fund L.P.</previousName>
        </edgarPreviousNameList>
        <entityType>Limited Partnership</entityType>
        <yearOfInc>
            <withinFiveYears>true</withinFiveYears>
            <value>2009</value>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Glenn</firstName>
                <lastName>Hill</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Margaret</firstName>
                <lastName>Conklin</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Thomas</firstName>
                <lastName>McGraw</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Shideh</firstName>
                <lastName>Theunissen</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>David</firstName>
                <lastName>Wezdenko</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>David</firstName>
                <lastName>Frame</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>c/o J.P. Morgan Private Invs. Int'l Inc.</street1>
                <street2>270 Park Avenue</street2>
                <city>New York</city>
                <stateOrCountry>NY</stateOrCountry>
                <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                <zipCode>10017</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification>Director of J.P. Morgan Private Investments International Inc., General Partner</relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Hedge Fund</investmentFundType>
                <is40Act>false</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Decline to Disclose</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
            <item>3C</item>
            <item>3C.7</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>0000950103-09-002690</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2009-10-05</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>true</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>100000</minimumInvestmentAccepted>
        <salesCompensationList>
            <recipient>
                <recipientName>J.P. Morgan Securities Inc.</recipientName>
                <recipientCRDNumber>79</recipientCRDNumber>
                <associatedBDName>None</associatedBDName>
                <associatedBDCRDNumber>None</associatedBDCRDNumber>
                <recipientAddress>
                    <street1>270 Park Avenue</street1>
                    <city>New York</city>
                    <stateOrCountry>NY</stateOrCountry>
                    <stateOrCountryDescription>NEW YORK</stateOrCountryDescription>
                    <zipCode>10017</zipCode>
                </recipientAddress>
                <statesOfSolicitationList>
                    <value>All States</value>
                </statesOfSolicitationList>
                <foreignSolicitation>false</foreignSolicitation>
            </recipient>
        </salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>Indefinite</totalOfferingAmount>
            <totalAmountSold>781095000</totalAmountSold>
            <totalRemaining>Indefinite</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>true</hasNonAccreditedInvestors>
            <numberNonAccreditedInvestors>1</numberNonAccreditedInvestors>
            <totalNumberAlreadyInvested>2</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>J.P. Morgan-BlackRock Opportunistic RMBS Master Fund L.P.</issuerName>
                <signatureName>/s/ Glenn Hill</signatureName>
                <nameOfSigner>Glenn Hill</nameOfSigner>
                <signatureTitle>Director of J.P. Morgan Private Investments Int'l Inc.</signatureTitle>
                <signatureDate>2010-04-14</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----