Thrombocytopenia is a frequent complication of primary antiphospholipid syndrome (PAPL) and has been attributed to antibodies directed against platelet glycoproteins (Gp) and also to antiphospholipid antibodies. We tested patients with PAPL with and without thrombocytopenia for specific antiplatelet autoantibodies. Platelet autoantibodies were detected by means of platelet immunoassays which included MAIPA with a panel of monoclonal antibodies directed against all the platelet Gps known to be possible targets for platelet autoantibodies. A high prevalence of serum platelet antibodies was found in patients with thrombocytopenia (73%, 11/15 patients) whereas antiplatelet antibody was detected in only one of the 10 control patients (P < 0.01). The antibodies mainly recognized GpIIbIIIa (n = 7), but also CD9 (n = 5), GpIaIIa (n = 4). GpIbIX (n = 3) and GpIV (n = 3). Platelet-Gp antibodies eluted from the platelet surface had the same reactivity as those found in the original sera from three of the four patients tested, whereas no anticardiolipin activity was found in the platelet eluates, suggesting the absence of cross-reactivity between anticardiolipin and antiplatelet antibodies. The MAIPA assay was also performed with F(ab')2 fragments obtained by pepsin digestion of serum IgG from four patients. The same results were obtained with F(ab')2 fragments and the original serum, demonstrating that platelet antibodies specifically bind in vivo to platelet Gps via their F(ab')2 fragments. Our results suggest a link between specific platelet antibodies and the thrombocytopenia of PAPL.