The binding of unstimulated and thrombin-stimulated platelets was studied with immobilized fibrinogen and fibrin on polystyrene. The amount of fibrinogen bound to the polystyrene support was 2 μg/tube, which represents 2.35 μg/cm2. Immobilized fibrin was obtained by adding thrombin (5 nM) to immobilized fibrinogen. The number of unstimulated 111ln-platelets bound to immobilized fibrinogen and fibrin was similar (3.2 ± 0.3 × 106 and 3.1 ± 0.4 × 106 platelets/μg fibrin-(ogen), respectively). The platelet binding steadily increased. In the first 2 min, the binding rate was 0.23 × 106 platelets/μg fibrinogen/min. The binding rate then increased rapidly and saturation was reached at 10 min. The extent of the adhesion of resting platelets to immobilized fibrinogen is about one half that of the same platelets stimulated with thrombin. In thrombin-stimulated 111In-platelets, the binding to immobilized fibrinogen and fibrin is time dependent, and saturation is reached at 5 min. The early rate of thrombin-stimulated platelet binding to fibrinogen is about twice that of binding to fibrin (1.25 and 0.74 × 106 platelets/μg fibrin(ogen)/min, respectively). In saturation conditions, 1 μg fibrinogen binds 5.7 ± 0.6 × 106 thrombin-stimulated platelets and 1 μg fibrin binds 4.6 ± 0.5 × 106 thrombin-stimulated platelets. Our results indicate that the rate of platelet aggregation is faster than fibrin formation, and the rate of fibrinogen-platelet binding is faster than that of fibrin-platelet binding. Therefore, after thrombin stimulation, the binding of platelets to fibrin must be secondary to the binding of platelets to fibrinogen.

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