IAC-1 binding to α₂β₁ on activated washed platelets is decreased compared with platelets activated in plasma and increased by binding of fibrinogen to α_IIbβ₃. Flow cytometric analysis of IAC-1 (A) or anti–P-selectin (B) binding to resting or CVX-stimulated platelets in plasma (PRP; □) or in buffer in the presence of increasing concentrations of fibrinogen (WP; ▪). All data are expressed as the mean ± SEM of at least 3 independent experiments. In all experiments, saturating concentrations of fluorescent antibodies were used.