Although plasmin can trigger strong platelet responses such as shape change and exocytosis of internal granules, limited platelet aggregation is induced by this proteinase, owing to its capacity to rapidly proteolyse secreted adhesive proteins. In this context, we have investigated the state of activation of the fibrinogen receptor, the integrin alpha IIb beta 3, on platelets exposed to plasmin. Following incubation with plasmin at 37 degrees C, washing, and resuspension, platelets exhibit a moderate, low-velocity aggregation when stirred in the presence of fibrinogen. Optimum aggregability is observed when platelets have been exposed to plasmin activity of approximately 0.5 CU/ml for 20 min, and aggregation is insensitive to the presence of antagonists such as prostaglandin (PG) E1 and apyrase. Plasmin-induced platelet aggregability is associated with the expression of active fibrinogen receptors on the cell surface, which, using a 125I-fibrinogen binding assay, can be quantified to approximately 2,300 molecules per platelet. Exposure of active alpha IIb beta 3 receptors appears to depend partially, but not totally on a metabolic activation and granule exocytosis at the time of incubation with plasmin. In contrast with alpha-thrombin, plasmin-induced activation of alpha IIb beta 3 is sustained and cannot be reversed by exposure of platelets to PGE1. Immunoblotting analysis of the receptor subunits shows no extensive proteolytic modification of alpha IIb beta 3 by plasmin, and only reveals a limited proteolysis of the aminoterminal domain of the alpha IIb subunit. In addition to their capacity to aggregate in the presence of fibrinogen alone, plasmin-treated platelets also show a potentiated aggregability in response to low doses of ADP.(ABSTRACT TRUNCATED AT 250 WORDS)