We have shown previously that the domain recognizing receptors on activated human platelets is located on the human fibrinogen .gamma. chain between residues 400 and 411 [Kloczewiak, M., Timmons, S., Lukas, T.J. and Hawigar, J. (1984) Biochemistry 23, 1767]. To study the correlation between the structure of this segment of the .gamma. chain and its reactivity toward receptors on ADP-activated human platelets, we designed a series of analogues containing replacements at 9 out of 12 positions. A double substitution of the normal His400-His401 sequence by Ala-Ala reduced the inhibitory potency of the dodecapeptide 3-fold. When Lys406 was replaced by Arg, the inhibitory potency of the dodecapeptide decreased 15 times. On the other hand, substitution of Ala408 with Arg increased the inhibitory potency of the dodecapeptide 6-fold. A drastic decrease in the reactivity of the dodecapeptide toward platelet receptors was observed when Val411 was replaced by leucine or cysteine or tyrosine. A 3-fold decrease in reactivity was noted when Val411 was substituted with phenylalanine. Amidation of the carboxy-terminal Val411 also produced a significant decrease in dodecapeptide reactivity. With seven residues (His400, His401, Leu402, Lys406, Gln407, and Val411) preserved, substitution of the intervening five amino acids with nonpolar leucine or polar serine, increasing or decreasing the hydrophobicity of the dodecapeptide, reduced more than 16-fold its inhibitory potency. Rabbit antibody Fab fragments directed against the human fibrinogen .gamma.-chain peptide encompassing residues 385-411 inhibited 50% of 125I-fibrinogen binding at a 2:1 stoichiometry with regard to 125I-fibrinogen. In vivo infusion of dodecapeptide with a native sequence into rabbit mesenteric artery caused reversible inhibition of hemostatic platelet thrombus formation.