During platelet activation, the glycoprotein (GP) complex IIb-IIIa (alpha 11b beta 3-integrin) changes its conformation, resulting in binding of adhesive proteins of RGD containing an amino acid sequence as well as in expression of new ligand-induced binding sites (LIBS) on the GPIIb-IIIa molecule. Like its F(ab)-fragments, the monoclonal antibody CRC54, whose epitope is located in the N-terminal part of the GPIIIa molecule, binds to no more than 10% of GPIIb-IIIa on the resting platelet surface. However, the binding of CRC54 increases considerably during activation of platelets by thrombin, platelet adhesion on plastic, GPIIb-IIIa interaction with RGDS-peptide as well as during dissociation of the complex in the presence of EDTA. These finding suggest that CRC54 is specifically directed against the LIBS epitope on the GPIIIa molecule. This epitope differs from those of other known conformation-dependent antibodies against GPIIb-IIIa (LIBS1, LIBS6, PMI-1, pl55 and p180), since those antibodies did not block the CRC54 binding to GPIIb-IIIa on the surface of adhering platelets. Unlike whole platelets, the binding of GPIIb-IIIa from lysates of platelets treated with Triton X-100 with immobilized CRC54 did not depend on the presence of the RGDS peptide. Under these conditions another anti-LIBS-antibody, p180 specifically directed against GPIIb, preserved its ability to discriminate the RGDS-occupied and resting conformations of GPIIb-IIIA. CRC54 and its F(ab) fragments induced platelet aggregation in both platelet-enriched plasma and in suspensions of washed platelets. CRC54 also stimulated the binding to platelets of GPIIb-IIIa ligand fibrinogen, labelled with 125I as well as adhesion of 51Cr-labelled platelets to immobilized ligands-fibrinogen and fibronectin. The CRC54-dependent aggregation was fully blocked by RGDS-peptide and antibody CRC64 inhibiting the GPIIb-IIIa binding to the ligands. However, the platelet activation inhibitor, prostaglandin EI, and the mixture of metabolic inhibitors, deoxyglucose-sodium azide, only party inhibited the CRC54-dependent aggregation. Incubation of platelets with CRC54 induced the binding to platelets of the anti-GPIIb LIBS antibody p180 and of the anti-GPIIb-IIIa activation-dependent antibody p155. The binding of GPIIb-IIIa from lysates of CRC54-treated platelets with immobilized p180 and p155 was also several times as high as that of GPIIb-IIIa from control platelet lysates. The data obtained indicate that the GPIIb-IIIa transition to the active state and its interaction with ligands induces conformational changes in the N-terminal part of GPIIIa and that the CRC54 binding to the N-terminal part of GPIIIa stimulates conformational changes in GPIIb-IIIa, complex interaction with ligands and platelet aggregation.