Abstract: Fibrinogen peptide derivatives which have biological activity as anticoagulants of blood having the dipeptide moiety prolylarginine or the tripeptide moiety glycylprolylarginine, e.g., L-prolyl-L-arginine benzyl ester p-toluenesulfonate trifluoroacetate, glycyl-L-prolyl-L-arginine benzyl ester p-toluenesulfonate trifluoroacetate, N-t-butyloxycarbonyl-L-prolyl-L-arginine benzyl ester p-toluenesulfonate methanolate, N-benzyloxycarbonyl-prolyl-L-arginine benzyl ester p-toluenesulfonate methanolate and N-t-butyloxycarbonylglycol-L-prolyl-L-arginine benzyl ester p-toluenesulfonate.