By means of immunoblotting and monoclonal antibody immobilization of platelet antigens (MAIPA) we have studied the specificity of antiplatelet antibodies in patients with antiphospholipid antibodies and thrombocytopenia defined as presence of anticardiolipin IgG and a platelet count below 100 x 10(9)/l. The study group consisted of 10 patients with systemic lupus erythematosus (SLE), 8 patients with primary anti-phospholipid syndrome (PAPS) and 16 patients with idiopathic thrombocytopenic purpura (ITP). The comparison group was formed by 17 patients with classical chronic ITP without anticardiolipin IgG. We identified the 80-100, 130-150 and 150-170 KD surface proteins that comigrate with GPIIIa, GPIIb and GPIb and a 50-70 KD cytoplasm band by immunoblot. In patients with classical chronic ITP, the prevalence of the antiplatelet antibodies against GPIIIa was 53% on immunoblot assay and 47% on MAIPA. In ITP patients who had also anti-phospholipid antibodies in serum, the percentage of reactivity to GPIIIa declined to 37% on immunoblot and 21% on MAIPA but it was not statistically different from the percentage observed in patients with classical ITP. Autoantibodies to platelet surface glycoproteins were almost absent in SLE and PAPS patients, who showed a significant prevalence (78%) of IgG reactivity to the 50-70 KD internal platelet protein which was frequently encountered also in patients with ITP and aPL (56%). Our study provides additional evidence that platelet antigens in patients with phospholipid-associated secondary immune thrombocytopenia are different from those of primary ITP, and that surface glycoproteins were not involved.