The antithrombotic effect of antiplatelet agents is principally due to their anti-aggregatory action, but these agents may also interfere with coagulation. We have investigated the effect of monoclonal antibodies (MAb) to platelet membrane glycoproteins (GP) IIb/IIIa and Ibalpha on thrombin generation. Antibodies to platelet membrane glycoprotein IIb/IIIa (RFGP56 and c7E3) were shown to inhibit platelet-mediated thrombin generation stimulated by both intrinsic and extrinsic methods. An antibody to GP Ibalpha (RFGP37) also inhibited thrombin generation in these systems. FITC-annexin V was used to determine the effect of these antibodies on the exposure of procoagulant phospholipids on the platelet membrane, and it was found that the anti-IIb/IIIa antibodies reduced this, whereas the anti-Ibalpha antibody caused an increase. We conclude that our monoclonal antibodies against platelet membrane glycoproteins IIb/IIIa and Ibalpha inhibit platelet dependent thrombin generation by different mechanisms.