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The high molecular weight form of the plasminogen activator urokinase (54kD) binds to specific receptor sites on the cell membrane of breast carcinomas by its inactive 'A' chain. The binding is of high affinity (range of dissociation constants: 5.6 x 10-II to 4 x 10 -10 mol and there were between 20 to 250fmol of binding sites per milligram of membrane protein) and equilibrium is reached in 60min. No competition for binding sites was observed with eqpidermal growth factor, tissue plasminogen activator or the low molecular weight form of urokinase (33 kD). Cross-linking experiments suggest that the receptor is a monomeric unit of molecular weight of 50kD. This binding site provides a mechanism for the incorporation of urokinase into the cell membrane.