The blockade of platelet integrin glycoprotein (GP) IIb/IIIa is a promising new antiplatelet strategy. The binding of ligands or of the ligand-mimetic peptide RGD causes a conformational change of GP IIb/IIIa from the nonactivated to the activated state. Because several blocking agents/inhibitors are ligand-mimetics, the current study evaluates whether these agents have the intrinsic property to activate GP IIb/IIIa. Fibrinogen binding to GP IIb/IIIa on platelets or on CHO cells expressing recombinant GP IIb/IIIa was evaluated by flow cytometry or 125I-labeled fibrinogen. Incubation with the monoclonal antibody (MoAb) fragment c7E3 (abciximab) results in fibrinogen binding to GP IIb/IIIa and in the access of ligand-induced binding sites. At low concentrations (0.01 to 0.1 microgram/mL), this intrinsic activating property of c7E3 can result in platelet aggregation. The disintegrin flavorodin and the RGD analogue fradafiban also induce fibrinogen binding, whereas the blocking MoAbs 2G12 and P2 and the activation-specific MoAb PAC-1 do not. Aspirin and indomethacin cannot block c7E3-induced fibrinogen binding to GP IIb/IIIa, but can inhibit c7E3-induced platelet aggregation. Thus, we conclude that GP IIb/IIIa inhibitors can demonstrate an intrinsic activating property, which can result in fibrinogen binding to GP IIb/IIIa and consequently in platelet aggregation. Cyclooxygenase inhibitors can inhibit platelet aggregation caused by GP IIb/IIIa inhibitors. Further studies will have to evaluate the clinical relevance of the potential intrinsic activating property of GP IIb/IIIa inhibitors and define consequences for the future drug development and evaluation of these potent antiplatelet agents.