The mechanisms by which lupus anticoagulant and antiphospholipid autoantibodies cause hemostatic abnormalities in patients with systemic lupus erythematosus (SLE) are poorly understood. We have approached this problem by investigating the binding and functional effects of human hybridoma lupus anticoagulant, anti-phospholipid and anti-DNA autoantibodies derived from SLE patients on platelets and endothelial cells. Most lupus anticoagulant antibodies did not bind to intact platelets and endothelial cells in vitro, while many antiphospholipid and anti-DNA antibodies were reactive. A comparison of SLE and normal-derived autoantibodies demonstrated that platelet-binding autoantibodies derived from SLE patients exhibited greater antigen specificity and platelet cytotoxicity than similar antibodies derived from normal individuals. By Western blotting analysis, many SLE-derived polyspecific antibodies reacted specifically with individual platelet proteins, whereas normal-derived polyspecific antibodies did not. One SLE-derived antibody, 9604 was found to react with ADP-activated platelets but not resting platelets. The reactive components in platelets were reducible polypeptides of approximately 200,000 and 32,000 molecular weight. These data suggest that some SLE autoantibodies may be able to interact with platelets and result in cell lysis or dysfunction in vivo.