Using a solid phase radioimmunoassay to detect anticardiolipin antibodies, the crossreactivity of these antibodies with phospholipids and DNA was tested in 10 patients with autoimmune disorders and 1 patient with secondary syphilis. The crossreactivity of anti-ds DNA and anti-ss DNA antibodies with cardiolipin was tested in 10 additional patients with SLE. In the 10 sera tested with high anticardiolipin antibody levels, there was marked inhibition of anticardiolipin antibody activity by negatively charged phospholipids. The zwitterionic phospholipids inhibited anticardiolipin antibody activity in some sera, and in instances where this occurred, inhibition was less than that obtained for the negatively charged phospholipids. There was minimal inhibition of anticardiolipin antibody by ss DNA in only 2 of 10 sera tested and no inhibition by ds DNA in any sera tested. Conversely, cardiolipin inhibited anti-ss DNA antibody activity in 2 of 10 sera, and anti-ds DNA antibody activity in 2 of 10 sera tested. There was no correlation between anticardiolipin antibody levels and either anti-ds DNA or anti-ss DNA antibody levels. These results suggest that antiphospholipid and anti-DNA antibodies are largely separate groups of antibodies. On the basis of these studies, we suggest that there may be various groups of antiphospholipid antibodies in patients with SLE and other autoimmune disorders.