Guanidinophenyl derivatives of pyrazole have been synthesized. Their inhibitory effects on (i) bovine trypsin, bovine thrombin, porcine pancreatic kallikrein catalyzed hydrolysis of p-nitro anilide of N alpha -benzoyl-arginine and (ii) blood coagulation and platelet aggregation, were investigated. The kinetic behaviour of all compounds conformed to that of a reversible competitive inhibition pattern, and they were also found to act in vitro as inhibitors of platelet aggregation induced by ADP.