A large number of similarities have previously been noted between the blood and milk clotting phenomena [Jollès, P. (1975) Mol. Cell. Biochem. 7, 73–85; Jollès, P. & Henschen, A. (1982) Trends Biochem. Sci. 7, 325–328]: some analogous features have also been found between fibrinogen and k-casein. In this connection, the effect of a natural and a synthetic peptide derived from k-casein on platelet function was studied: the undecapeptide Met-Ala-Ile-Pro-Pro-Lys-Lys-Asn-Gln-Asp-Lys (residues 106 → 116 of cow k-casein) inhibited both aggregation of ADP-treated platelets and binding of 125I-fibrinogen to ADP-treated platelets: its behaviour was similar to that of the structurally related C-terminal dodecapeptide of human fibrinogen γ-chain.