Platelet activation is accompanied by the appearance on the platelet surface of approximately 45,000 receptor sites for fibrinogen. The binding of fibrinogen to these receptors is required for platelet aggregation. Although it is established that the fibrinogen receptor is localized to a heterodimer complex of the membrane glycoproteins, IIb and IIIa, little is known about the changes in this complex during platelet activation that result in the expression of the receptor. In the present studies, we have developed and characterized a murine monoclonal anti-platelet antibody, designated PAC-1, that binds to activated platelets, but not to unstimulated platelets. PAC-1 is a pentameric IgM that binds to agonist-stimulated platelets with an apparent Kd of 5 nM. Binding to platelets is dependent on extracellular Ca2+ (KCa = 0.4 microM) but is not dependent on platelet secretion. Platelets stimulated with ADP or epinephrine bind 10,000-15,000 125I-PAC-1 molecules/platelet while platelets stimulated with thrombin bind 20,000-25,000 molecules/platelet. Several lines of evidence indicate that PAC-1 is specific for the glycoprotein IIb.IIIa complex. First, PAC-1 binds specifically to the IIb.IIIa complex on Western blots. Second, PAC-1 does not bind to thrombasthenic platelets or to platelets preincubated with ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid at 37 degrees C, both of which lack the intact IIb.IIIa complex. Third, PAC-1 competitively inhibits the binding of 125I-A2A9, and IgG monoclonal antibody that is specific for the IIb.IIIa complex. Fourth, the antibody inhibits fibrinogen-mediated platelet aggregation. These data demonstrate that PAC-1 recognizes an epitope on the IIb.IIIa complex that is located near the platelet fibrinogen receptor. Platelet activation appears to cause a Ca2+-dependent change involving the glycoprotein IIb.IIIa complex that exposes the fibrinogen receptor and, at the same time, the epitope for PAC-1.