The thienopyridines, ticlopidine and PCR 4099, inhibit ex vivo aggregation in response to ADP and other agonists. It has been shown that ticlopidine induces a functional defect in the binding of fibrinogen to its platelet membrane receptor. We have studied the effects on platelet functions of PCR 4099 in rat and in man. The aim of the study was to check the possibility of a direct modification of the fibrinogen binding site on the GP IIb-IIIa complex. Washed platelet suspensions were used for aggregation and fibrinogen binding studies. Platelet lysates were submitted to SDS-polyacrylamide gel electrophoresis, crossed immunoelectrophoresis and immunoprecipitation. We found that administration of PCR 4099 inhibited selectively and irreversibly ADP-induced aggregation. Although the effect of ADP on aggregation was blocked, PCR 4099 did not modify ADP-induced shape change. Only the effects of low concentrations of thrombin on platelet aggregation were inhibited. Fibrinogen binding was dramatically inhibited in rat and in man when platelets were stimulated with ADP and low concentrations of thrombin. At high concentration of thrombin there still remained a part of fibrinogen binding inhibition although aggregation was not impaired. Electrophoretic and immunoelectrophoretic studies showed no difference before and after treatment by PCR 4099. In particular, the GP IIb-IIIa-complex was not dissociated, its electrophoretic mobility was not changed and three monoclonal anticomplex antibodies recognized it in the same manner before and after treatment. We conclude that PCR 4099 selectively inhibits the ADP aggregation pathway and that the inhibition of fibrinogen binding is probably not due to a direct modification of the GP IIb-IIIa complex.