Non-specific binding in an ELISA test for antibodies to cardiolipin (ACA) was investigated in sera from patients with primary biliary cirrhosis, rheumatological disorders, and repeated abortion. Binding to wells without phospholipid was most frequent in ELISA assay for IgM class ACA and correlated with levels of serum polyclonal IgM in patients with repeated abortion and rheumatologic disorders (r = 0.79, 0.76). Using added polyclonal IgM we demonstrated that the increase in non-specific binding was most significant when levels of ACA were low; subtraction of binding to the unoccupied well over-corrected specific antibody estimates when a high affinity antibody was present. Absorption studies on three sera suggest that this 'non-specific' binding includes low affinity antibody binding. Elevated levels of polyclonal IgM in sera from patients with repeated abortion may account for some reports of elevated IgM class antibodies in other ELISA assays. The prevalence of a positive test for ACA in a sequential series of 412 such patients was reduced to 6.1% (95% confidence limits 4-9%) after subtraction of non-specific binding.