Participation of fibrinogen in platelet aggregation is contingent upon the capacity of various stimuli to induce specific receptors for the molecule on the surface of the cell. The interaction of fibrinogen with this receptor results directly in platelet aggregation, and dissociation of fibrinogen is associated with disaggregation. While the role of exogenous fibrinogen in this process has been fully documented, the mechanisms which control the surface exposure of platelet fibrinogen are less understood. In the present study Fab fragments of antibodies monospecific for fibrinogen have been used to examine the surface expression of intracellular fibrinogen and its involvement in platelet aggregation. Radiolabelled Fab fragments did not interact with non-stimulated platelets but significant binding was observed when the cells were stimulated by ADP, thrombin, collagen and Ca ionophore A23187. Binding was specific for fibrinogen, was not observed with thrombasthenic platelets and was dependent upon the presence of extracellular calcium. With all stimuli tested, the binding of the Fab probe to platelets correlated with platelet secretion. At the following concentrations of stimuli: 30 microM ADP, 4 micrograms/ml collagen, 3 microM A23187 and 0.05 U/ml thrombin, the immune Fab fragments inhibited platelet aggregation. A monoclonal antibody to glycoprotein IIb/IIIa complex and a synthetic peptide gamma 400-411, that inhibited the interaction of plasma fibrinogen with platelets, did not inhibit the binding of 125I-FAB fragments. Taken together these results support the hypothesis that endogenous fibrinogen becomes surface-expressed during stimulation of the cell and can support platelet aggregation, particularly that induced by low concentrations of stimuli. The mechanism for the surface expression of platelet fibrinogen may be distinct from that for the binding of plasma fibrinogen.