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Fibrinogen binding to receptors on stimulated platelets is a prerequisite for platelet aggregation. To gain further insight into the role of fibrinogen in platelet aggregation and to identify the platelet fibrinogen receptor, we developed a monoclonal anti-platelet antibody that inhibited platelet aggregation. The purified antibody, designated A2A9, inhibited platelet aggregation stimulated by 10 microM ADP, 10 microM epinephrine, and thrombin at 1 unit/ml without inhibiting platelet shape change or platelet secretion. A2A9 was also a competitive inhibitor of fibrinogen binding to ADP-stimulated platelets. Fifty percent inhibition of fibrinogen binding occurred at 65 nM A2A9. Direct binding studies using radiolabeled A2A9 demonstrated 47,000 A2A9 binding sites on unstimulated platelets, with a dissociation constant of 60 nM. Platelets from two individuals with Glanzmann thrombasthenia bound essentially no A2A9 …