<?xml version="1.0" encoding="UTF-8"?>
<edgarSubmission xmlns="http://www.sec.gov/edgar/nmfp" xmlns:com="http://www.sec.gov/edgar/common" xmlns:invest="http://www.sec.gov/edgar/invest" xmlns:part1="http://www.sec.gov/edgar/nmfpfund" xmlns:part2="http://www.sec.gov/edgar/nmfpsecurities" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sec.gov/edgar/nmfp eis_NMFP_Submission.xsd">
  <submissionType>N-MFP</submissionType>
  <liveTestFlag>LIVE</liveTestFlag>


  <isThisElectronicCopyOfPaperFormat>N</isThisElectronicCopyOfPaperFormat>
  <DocumentPeriodEndDate>2011-05-31</DocumentPeriodEndDate>
  <EntityCentralIndexKey>0000915092</EntityCentralIndexKey>

  <seriesId>S000004289</seriesId>
  <totalClassesInSeries>1</totalClassesInSeries>
  <isThisFinalFiling>N</isThisFinalFiling>
  <isFundLiquidating>N</isFundLiquidating>
  <isFundMergingWithOrBeingAcquiredByAnotherFund>N</isFundMergingWithOrBeingAcquiredByAnotherFund>
  <hasFundAcquiredOrMergedWithAnotherFundSinceLastFiling>N</hasFundAcquiredOrMergedWithAnotherFundSinceLastFiling>
  <seriesLevelInformation>
    <part1:investmentAdviserList>
      <part1:adviser>
        <part1:adviserName>BlackRock Fund Advisors</part1:adviserName>
        <part1:adviserFileNumber>801-22609</part1:adviserFileNumber>
      </part1:adviser>
    </part1:investmentAdviserList>
    <part1:independentPublicAccountant>
      <part1:name>PricewaterhouseCoopers </part1:name>
      <part1:city>New York</part1:city>
      <part1:state>NY</part1:state>
    </part1:independentPublicAccountant>
    <part1:administratorList>
      <part1:administrator>BlackRock Institutional Trust Company, N.A.</part1:administrator>
    </part1:administratorList>
    <part1:transferAgentList>
      <part1:transferAgent>
        <part1:name>State Street</part1:name>
        <part1:EntityCentralIndexKey>0001145958</part1:EntityCentralIndexKey>
        <part1:fileNumber>85-11329</part1:fileNumber>
      </part1:transferAgent>
    </part1:transferAgentList>
    <part1:isThisFeederFund>N</part1:isThisFeederFund>
    <part1:isThisMasterFund>Y</part1:isThisMasterFund>
    <part1:feederFundList>
      <part1:feederFund>
        <part1:EntityCentralIndexKey>0000893818</part1:EntityCentralIndexKey>
        <part1:EntityRegistrantName>BlackRock Cash Funds: Government</part1:EntityRegistrantName>
        <part1:ContainedFileInformationFileNumber>033-54126</part1:ContainedFileInformationFileNumber>
        <part1:seriesIdentifier>S000004274</part1:seriesIdentifier>
      </part1:feederFund>
    </part1:feederFundList>
    <part1:isThisSeriesPrimarilyUsedToFundInsuranceCompanySeperateAccounts>N</part1:isThisSeriesPrimarilyUsedToFundInsuranceCompanySeperateAccounts>
    <part1:InvestmentTypeDomain>Government/Agency</part1:InvestmentTypeDomain>
    <part1:dollarWeightedAveragePortfolioMaturity>1</part1:dollarWeightedAveragePortfolioMaturity>
    <part1:dollarWeightedAverageLifeMaturity>1</part1:dollarWeightedAverageLifeMaturity>
    <part1:AvailableForSaleSecuritiesAmortizedCost>13512000.00</part1:AvailableForSaleSecuritiesAmortizedCost>
    <part1:OtherAssets>950.24</part1:OtherAssets>
    <part1:Liabilities>6167.29</part1:Liabilities>
    <part1:AssetsNet>13506782.95</part1:AssetsNet>
    <part1:MoneyMarketSevenDayYield>0.0000</part1:MoneyMarketSevenDayYield>
    <part1:seriesShadowPrice>
      <part1:netValuePerShareIncludingCapitalSupportAgreement>0.0000</part1:netValuePerShareIncludingCapitalSupportAgreement>
      <part1:dateCalculatedFornetValuePerShareIncludingCapitalSupportAgreement>2011-05-31</part1:dateCalculatedFornetValuePerShareIncludingCapitalSupportAgreement>
      <part1:netValuePerShareExcludingCapitalSupportAgreement>0.0000</part1:netValuePerShareExcludingCapitalSupportAgreement>
      <part1:dateCalculatedFornetValuePerShareExcludingCapitalSupportAgreement>2011-05-31</part1:dateCalculatedFornetValuePerShareExcludingCapitalSupportAgreement>
    </part1:seriesShadowPrice>
  </seriesLevelInformation>
  <classLevelInformationList>
    <part1:classLevelInformation>
      <part1:classId>C000012011</part1:classId>
      <part1:minInitialInvestment>1.00</part1:minInitialInvestment>
      <part1:netAssetsOfClass>13506782.95</part1:netAssetsOfClass>
      <part1:netAssetValuePerShare>1.00</part1:netAssetValuePerShare>
      <part1:netShareholderFlowActivityForMonthEnded>-832043.69</part1:netShareholderFlowActivityForMonthEnded>
      <part1:grossSubscriptionsForMonthEnded>631335.01</part1:grossSubscriptionsForMonthEnded>
      <part1:grossRedemptionsForMonthEnded>1463378.70</part1:grossRedemptionsForMonthEnded>
      <part1:sevenDayNetYield>0.0000</part1:sevenDayNetYield>
      <part1:classShadowPrice>
        <part1:netAssetValuePerShareIncludingCapitalSupportAgreement>
          <part1:value>0.0000</part1:value>
          <part1:dateAsOfWhichValueWasCalculated>2011-05-31</part1:dateAsOfWhichValueWasCalculated>
        </part1:netAssetValuePerShareIncludingCapitalSupportAgreement>
        <part1:netAssetValuePerShareExcludingCapitalSupportAgreement>
          <part1:value>0.0000</part1:value>
          <part1:dateAsOfWhichValueWasCalculated>2011-05-31</part1:dateAsOfWhichValueWasCalculated>
        </part1:netAssetValuePerShareExcludingCapitalSupportAgreement>
      </part1:classShadowPrice>
    </part1:classLevelInformation>
  </classLevelInformationList>
  <scheduleOfPortfolioSecuritiesList>
    <part2:scheduleOfPortfolioSecurities>
      <part2:InvestmentIssuer>DEUTSCHE BANK SECURITIES INC.</part2:InvestmentIssuer>
      <part2:InvestmentTitle>TRI-PARTY DEUTSCHE BANK SECURITIES 0.130000%</part2:InvestmentTitle>
      <part2:InvestmentIdentifier>BRSC76KU5</part2:InvestmentIdentifier>
      <part2:EntityCentralIndexKey>0001074042</part2:EntityCentralIndexKey>
      <part2:InvestmentTypeDomain>Government Agency Repurchase Agreement</part2:InvestmentTypeDomain>
      <part2:isFundTreatingAsAcquisitionUnderlyingSecurities>Y</part2:isFundTreatingAsAcquisitionUnderlyingSecurities>
      <part2:repurchaseAgreementList>
        <part2:RepurchaseAgreement>
          <part2:InvestmentIssuer>FANNIE MAE</part2:InvestmentIssuer>
          <part2:InvestmentMaturityDate>
            <invest:date>2038-08-01</invest:date>
          </part2:InvestmentMaturityDate>
          <part2:CR>7.000000</part2:CR>
          <part2:InvestmentTypeDomain>Government Agency Debt</part2:InvestmentTypeDomain>
          <part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>3000000.00</part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>
          <part2:CashCollateralForBorrowedSecurities>3090000.35</part2:CashCollateralForBorrowedSecurities>
        </part2:RepurchaseAgreement>
      </part2:repurchaseAgreementList>
      <part2:rating>First Tier Security</part2:rating>
      <part2:designatedNRSROList>
        <part2:designatedNRSRO>
          <part2:RAN>N/A</part2:RAN>
          <part2:R>N/A</part2:R>
        </part2:designatedNRSRO>
      </part2:designatedNRSROList>
      <part2:InvestmentMaturityDate>2011-06-01</part2:InvestmentMaturityDate>
      <part2:finalLegalInvestmentMaturityDate>2011-06-01</part2:finalLegalInvestmentMaturityDate>
      <part2:doesSecurityHaveDemandFeature>N</part2:doesSecurityHaveDemandFeature>
      <part2:doesSecurityHaveGuarantee>N</part2:doesSecurityHaveGuarantee>
      <part2:doesSecurityHaveEnhancementsOnWhichFundRelying>N</part2:doesSecurityHaveEnhancementsOnWhichFundRelying>
      <part2:InvestmentOwnedBalancePrincipalAmount>3000000.00</part2:InvestmentOwnedBalancePrincipalAmount>
      <part2:AvailableForSaleSecuritiesAmortizedCost>3000000.00</part2:AvailableForSaleSecuritiesAmortizedCost>
      <part2:InvestmentOwnedPercentOfNetAssets>0.2221</part2:InvestmentOwnedPercentOfNetAssets>
      <part2:isThisIlliquidSecurity>N</part2:isThisIlliquidSecurity>
      <part2:InvestmentOwnedAtFairValue>3000000.00</part2:InvestmentOwnedAtFairValue>
      <part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>3000000.00</part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>
    </part2:scheduleOfPortfolioSecurities>
    <part2:scheduleOfPortfolioSecurities>
      <part2:InvestmentIssuer>GOLDMAN SACHS &amp; CO</part2:InvestmentIssuer>
      <part2:InvestmentTitle>TRI-PARTY GOLDMAN SACHS &amp; CO. 0.130000%</part2:InvestmentTitle>
      <part2:InvestmentIdentifier>BRSC76KB7</part2:InvestmentIdentifier>
      <part2:EntityCentralIndexKey>0000769993</part2:EntityCentralIndexKey>
      <part2:InvestmentTypeDomain>Government Agency Repurchase Agreement</part2:InvestmentTypeDomain>
      <part2:isFundTreatingAsAcquisitionUnderlyingSecurities>Y</part2:isFundTreatingAsAcquisitionUnderlyingSecurities>
      <part2:repurchaseAgreementList>
        <part2:RepurchaseAgreement>
          <part2:InvestmentIssuer>Ginnie Mae REMIC Trust</part2:InvestmentIssuer>
          <part2:InvestmentMaturityDate>
            <invest:date>2039-12-20</invest:date>
          </part2:InvestmentMaturityDate>
          <part2:CR>6.250000</part2:CR>
          <part2:InvestmentTypeDomain>Government Agency Debt</part2:InvestmentTypeDomain>
          <part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>2000000.00</part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>
          <part2:CashCollateralForBorrowedSecurities>2060000.72</part2:CashCollateralForBorrowedSecurities>
        </part2:RepurchaseAgreement>
      </part2:repurchaseAgreementList>
      <part2:rating>First Tier Security</part2:rating>
      <part2:designatedNRSROList>
        <part2:designatedNRSRO>
          <part2:RAN>N/A</part2:RAN>
          <part2:R>N/A</part2:R>
        </part2:designatedNRSRO>
      </part2:designatedNRSROList>
      <part2:InvestmentMaturityDate>2011-06-01</part2:InvestmentMaturityDate>
      <part2:finalLegalInvestmentMaturityDate>2011-06-01</part2:finalLegalInvestmentMaturityDate>
      <part2:doesSecurityHaveDemandFeature>N</part2:doesSecurityHaveDemandFeature>
      <part2:doesSecurityHaveGuarantee>N</part2:doesSecurityHaveGuarantee>
      <part2:doesSecurityHaveEnhancementsOnWhichFundRelying>N</part2:doesSecurityHaveEnhancementsOnWhichFundRelying>
      <part2:InvestmentOwnedBalancePrincipalAmount>2000000.00</part2:InvestmentOwnedBalancePrincipalAmount>
      <part2:AvailableForSaleSecuritiesAmortizedCost>2000000.00</part2:AvailableForSaleSecuritiesAmortizedCost>
      <part2:InvestmentOwnedPercentOfNetAssets>0.1481</part2:InvestmentOwnedPercentOfNetAssets>
      <part2:isThisIlliquidSecurity>N</part2:isThisIlliquidSecurity>
      <part2:InvestmentOwnedAtFairValue>2000000.00</part2:InvestmentOwnedAtFairValue>
      <part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>2000000.00</part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>
    </part2:scheduleOfPortfolioSecurities>
    <part2:scheduleOfPortfolioSecurities>
      <part2:InvestmentIssuer>MERRILL LYNCH PIERCE FENNER &amp; SMITH INC</part2:InvestmentIssuer>
      <part2:InvestmentTitle>TRI-PARTY MERRILL LYNCH, PIERCE, F 0.140000%</part2:InvestmentTitle>
      <part2:InvestmentIdentifier>BRSC76JS2</part2:InvestmentIdentifier>
      <part2:EntityCentralIndexKey>0000728612</part2:EntityCentralIndexKey>
      <part2:InvestmentTypeDomain>Government Agency Repurchase Agreement</part2:InvestmentTypeDomain>
      <part2:isFundTreatingAsAcquisitionUnderlyingSecurities>Y</part2:isFundTreatingAsAcquisitionUnderlyingSecurities>
      <part2:repurchaseAgreementList>
        <part2:RepurchaseAgreement>
          <part2:InvestmentIssuer>FREDDIE MAC</part2:InvestmentIssuer>
          <part2:InvestmentMaturityDate>
            <invest:date>2041-05-01</invest:date>
          </part2:InvestmentMaturityDate>
          <part2:CR>3.226999</part2:CR>
          <part2:InvestmentTypeDomain>Government Agency Debt</part2:InvestmentTypeDomain>
          <part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>3000000.00</part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>
          <part2:CashCollateralForBorrowedSecurities>3060000.69</part2:CashCollateralForBorrowedSecurities>
        </part2:RepurchaseAgreement>
      </part2:repurchaseAgreementList>
      <part2:rating>First Tier Security</part2:rating>
      <part2:designatedNRSROList>
        <part2:designatedNRSRO>
          <part2:RAN>N/A</part2:RAN>
          <part2:R>N/A</part2:R>
        </part2:designatedNRSRO>
      </part2:designatedNRSROList>
      <part2:InvestmentMaturityDate>2011-06-01</part2:InvestmentMaturityDate>
      <part2:finalLegalInvestmentMaturityDate>2011-06-01</part2:finalLegalInvestmentMaturityDate>
      <part2:doesSecurityHaveDemandFeature>N</part2:doesSecurityHaveDemandFeature>
      <part2:doesSecurityHaveGuarantee>N</part2:doesSecurityHaveGuarantee>
      <part2:doesSecurityHaveEnhancementsOnWhichFundRelying>N</part2:doesSecurityHaveEnhancementsOnWhichFundRelying>
      <part2:InvestmentOwnedBalancePrincipalAmount>3000000.00</part2:InvestmentOwnedBalancePrincipalAmount>
      <part2:AvailableForSaleSecuritiesAmortizedCost>3000000.00</part2:AvailableForSaleSecuritiesAmortizedCost>
      <part2:InvestmentOwnedPercentOfNetAssets>0.2221</part2:InvestmentOwnedPercentOfNetAssets>
      <part2:isThisIlliquidSecurity>N</part2:isThisIlliquidSecurity>
      <part2:InvestmentOwnedAtFairValue>3000000.00</part2:InvestmentOwnedAtFairValue>
      <part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>3000000.00</part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>
    </part2:scheduleOfPortfolioSecurities>
    <part2:scheduleOfPortfolioSecurities>
      <part2:InvestmentIssuer>MORGAN STANLEY &amp; CO. INC</part2:InvestmentIssuer>
      <part2:InvestmentTitle>TRI-PARTY MORGAN STANLEY &amp; CO INC 0.100000%</part2:InvestmentTitle>
      <part2:InvestmentIdentifier>BRSC76JY9</part2:InvestmentIdentifier>
      <part2:EntityCentralIndexKey>0001359291</part2:EntityCentralIndexKey>
      <part2:InvestmentTypeDomain>Treasury Repurchase Agreement</part2:InvestmentTypeDomain>
      <part2:isFundTreatingAsAcquisitionUnderlyingSecurities>Y</part2:isFundTreatingAsAcquisitionUnderlyingSecurities>
      <part2:repurchaseAgreementList>
        <part2:RepurchaseAgreement>
          <part2:InvestmentIssuer>UNITED STATES TREAS BILLS</part2:InvestmentIssuer>
          <part2:InvestmentMaturityDate>
            <invest:date>2012-02-09</invest:date>
          </part2:InvestmentMaturityDate>
          <part2:CR>0.000000</part2:CR>
          <part2:InvestmentTypeDomain>Treasury Debt</part2:InvestmentTypeDomain>
          <part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>2512000.00</part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>
          <part2:CashCollateralForBorrowedSecurities>2562276.85</part2:CashCollateralForBorrowedSecurities>
        </part2:RepurchaseAgreement>
      </part2:repurchaseAgreementList>
      <part2:rating>First Tier Security</part2:rating>
      <part2:designatedNRSROList>
        <part2:designatedNRSRO>
          <part2:RAN>N/A</part2:RAN>
          <part2:R>N/A</part2:R>
        </part2:designatedNRSRO>
      </part2:designatedNRSROList>
      <part2:InvestmentMaturityDate>2011-06-01</part2:InvestmentMaturityDate>
      <part2:finalLegalInvestmentMaturityDate>2011-06-01</part2:finalLegalInvestmentMaturityDate>
      <part2:doesSecurityHaveDemandFeature>N</part2:doesSecurityHaveDemandFeature>
      <part2:doesSecurityHaveGuarantee>N</part2:doesSecurityHaveGuarantee>
      <part2:doesSecurityHaveEnhancementsOnWhichFundRelying>N</part2:doesSecurityHaveEnhancementsOnWhichFundRelying>
      <part2:InvestmentOwnedBalancePrincipalAmount>2512000.00</part2:InvestmentOwnedBalancePrincipalAmount>
      <part2:AvailableForSaleSecuritiesAmortizedCost>2512000.00</part2:AvailableForSaleSecuritiesAmortizedCost>
      <part2:InvestmentOwnedPercentOfNetAssets>0.1860</part2:InvestmentOwnedPercentOfNetAssets>
      <part2:isThisIlliquidSecurity>N</part2:isThisIlliquidSecurity>
      <part2:InvestmentOwnedAtFairValue>2512000.00</part2:InvestmentOwnedAtFairValue>
      <part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>2512000.00</part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>
    </part2:scheduleOfPortfolioSecurities>
    <part2:scheduleOfPortfolioSecurities>
      <part2:InvestmentIssuer>RBS SECURITIES INC</part2:InvestmentIssuer>
      <part2:InvestmentTitle>TRI-PARTY RBS SECURITIES INC. 0.130000%</part2:InvestmentTitle>
      <part2:InvestmentIdentifier>BRSC76L42</part2:InvestmentIdentifier>
      <part2:EntityCentralIndexKey>0000808379</part2:EntityCentralIndexKey>
      <part2:InvestmentTypeDomain>Government Agency Repurchase Agreement</part2:InvestmentTypeDomain>
      <part2:isFundTreatingAsAcquisitionUnderlyingSecurities>Y</part2:isFundTreatingAsAcquisitionUnderlyingSecurities>
      <part2:repurchaseAgreementList>
        <part2:RepurchaseAgreement>
          <part2:InvestmentIssuer>FNAR AH9346</part2:InvestmentIssuer>
          <part2:InvestmentMaturityDate>
            <invest:date>2041-04-01</invest:date>
          </part2:InvestmentMaturityDate>
          <part2:CR>3.198000</part2:CR>
          <part2:InvestmentTypeDomain>Government Agency Debt</part2:InvestmentTypeDomain>
          <part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>3000000.00</part2:AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts>
          <part2:CashCollateralForBorrowedSecurities>3064463.10</part2:CashCollateralForBorrowedSecurities>
        </part2:RepurchaseAgreement>
      </part2:repurchaseAgreementList>
      <part2:rating>Unrated Security</part2:rating>
      <part2:designatedNRSROList>
        <part2:designatedNRSRO>
          <part2:RAN>N/A</part2:RAN>
          <part2:R>N/A</part2:R>
        </part2:designatedNRSRO>
      </part2:designatedNRSROList>
      <part2:InvestmentMaturityDate>2011-06-01</part2:InvestmentMaturityDate>
      <part2:finalLegalInvestmentMaturityDate>2011-06-01</part2:finalLegalInvestmentMaturityDate>
      <part2:doesSecurityHaveDemandFeature>N</part2:doesSecurityHaveDemandFeature>
      <part2:doesSecurityHaveGuarantee>N</part2:doesSecurityHaveGuarantee>
      <part2:doesSecurityHaveEnhancementsOnWhichFundRelying>N</part2:doesSecurityHaveEnhancementsOnWhichFundRelying>
      <part2:InvestmentOwnedBalancePrincipalAmount>3000000.00</part2:InvestmentOwnedBalancePrincipalAmount>
      <part2:AvailableForSaleSecuritiesAmortizedCost>3000000.00</part2:AvailableForSaleSecuritiesAmortizedCost>
      <part2:InvestmentOwnedPercentOfNetAssets>0.2221</part2:InvestmentOwnedPercentOfNetAssets>
      <part2:isThisIlliquidSecurity>N</part2:isThisIlliquidSecurity>
      <part2:InvestmentOwnedAtFairValue>3000000.00</part2:InvestmentOwnedAtFairValue>
      <part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>3000000.00</part2:valueOfSecurityExcludingValueOfCapitalSupportAgreement>
    </part2:scheduleOfPortfolioSecurities>
  </scheduleOfPortfolioSecuritiesList>
</edgarSubmission>
