Tetrafibricin; a nonpeptidic fibrinogen inhibitor from microbial origin, showed potent antiaggregation activities on human platelet aggregation induced by either ADP, thrombin or collagen (IC50s = 5.6, 7.6 and 11 microM, respectively) in platelet rich plasma. The ability to inhibit aggregation in platelets treated with chymotrypsin confirmed the GPIIb/IIIa blockage of tetrafibricin. Tetrafibricin blocked the release of serotonin induced by ADP but it did not block the release reaction induced by thrombin. When added to platelets formerly aggregated with ADP, tetrafibricin caused rapid and complete deaggregation. As for the selectivity among other Arg-Gly-Asp -dependent integrins, tetrafibricin seems to be more specific for glycoprotein (GP) IIb/IIIa than RGDS is. This is because it had no effect on adhesion of bovine aortic endothelial cells to RGD-containing proteins. Tetrafibricin is the first nonpeptidic fibrinogen receptor inhibitor that may be valuable for the study on platelet aggregation inhibitors.