A haemolytic assay was used to test the complement fixation ability of 16 serum samples with high concentrations of anti-cardiolipin antibodies. Fourteen patients had clinical complications usually associated with these antibodies--namely, recurrent abortions, thrombosis, or thrombocytopenia. Complement fixation by anticardiolipin antibodies was shown in only four of these patients and was not directly related to the antibody concentration. Because anticardiolipin antibodies in most of these patients did not activate the complement pathway it is unlikely that the complement cascade has an important role in the clinical complications associated with these antibodies.