Spleen cells of BALB/c mice, immunized with fragments Y of normal human fibrinogen, were fused with P3 x 63 Ag 8653 myeloma cells. A clone was found which produces monoclonal antibodies (Mab-Y18) of the IgM?? type. Mab-Y18 is immunoreactive with normal human fibrinogen, and its fragments X, Y, N-terminal disulphide knot, A??-chain, and A?? stretch 1-51. The immunoreactivity with these same fragments disappears upon treatment with thrombin or arvin. This strongly suggests that fibrinopeptide A is an essential component of the Mab-Y18 epitope. This is supported by the finding that Mab-Y18 prolongs the thrombin and arvin clotting times of human fibrinogen by inhibition of the fibrinopeptide A release. More detailed information about the nature of the Mab-Y18 epitope was obtained from studies with genetic variants of human fibrinogen (especially fibrinogen Metz) and with fibrinogens from other mammalian species. These studies show that amino acid residue A??16 (arginine) of fibrinopeptide A is essential for the Mab-Y18 epitope. Mab-Y18 does not react with free fibrinopeptide A.Chemicals/CAS: ancrod, 9046-56-4; fibrin, 9001-31-4; fibrinogen, 9001-32-5; fibrinopeptide A, 25422-31-5; thrombin, 9002-04-4; Antibodies, Monoclonal; Fibrin Fibrinogen Degradation Products; fibrinogen fragment Y; Fibrinogen, 9001-32-5; Fibrinopeptide A, 25422-31-5