The bradykinin (BK) fragment (1-5) (RPPGF) is among the most stable of naturally occurring metabolites. Bradykinin (1-5) may be used as a marker for BK production in vivo. It is known that an intact Arg residue in the C-terminus is required for certain biological activities. BK 1-5 is the smallest peptide that can inhibit α-thrombin-induced platelet aggregation and secretion and calcium mobilization. Bradykinin (1-5) also prevents α-thrombin from cleaving the thrombin receptor peptide, NATLDPRSFLLR, between arginine and serine. Such antithrombin activities of BK 1-5 may contribute to the cardioprotective nature of kinins.