The binding of fibrinogen to its GPIIb-IIIa receptor is divalent-cation dependent. In addition to Ca+2 and Mg+2, Mn+2 has been shown to modulate adhesive protein interactions with integrins. This study examined the effect of Mn+2 on fibrinogen interactions with intact platelets. Compared with that of control platelets in buffer containing 1 mmol/L Mg+2, fibrinogen binding to adenosine diphosphate- or thrombin-stimulated platelets decreased 23% +/- 12% and 15% +/- 9% (mean +/- SD, n = 4), respectively, after addition of 1 mmol/L Mn+2. No change in binding affinity was noted, but the stability of platelet-fibrinogen interactions was diminished markedly. Ethylenediaminetetraacetic acid dissociated 68% +/- 8% of fibrinogen bound to ADP-treated platelets (p < 0.05) during a 60-minute incubation with fibrinogen and 1 mmol/L Mn+2, compared with 40% +/- 13% of fibrinogen bound to control platelets and 29% +/- 8% of fibrinogen bound in the presence of Ca+2 (mean +/- SD, n = 6). Mn+2 also diminished the stabilization of fibrinogen interaction with thrombin-stimulated platelets and inhibited the recovery of bound fibrinogen with the Triton X-100 (Union Carbide Corp., Danbury, Conn.) insoluble cytoskeleton. Only 31% +/- 10% of fibrinogen bound to thrombin-stimulated platelets for 60 minutes in the presence of Mn+2 associated with the cytoskeleton (p < 0.05), compared with 61% +/- 14% and 75% +/- 20% of fibrinogen bound to control platelets incubated with and without Ca+2, respectively. Mn+2 further inhibited large adenosine diphosphate- or thrombin-induced platelet aggregate formation and reduced the ability of platelets to retract fibrin clots. These data suggest that Mn+2 alters GPIIb-IIIa function relative to native fibrinogen and support a role for the stabilization of platelet-fibrinogen interactions in platelet aggregation and clot retraction.