IgG fractions were purified on Sepharose anti-human IgG column from eight sera of healthy donors, having no anti-cardiolipin (aCL) activity as measured by anti-cardiolipin ELISA assay (aCL-ELISA). All the IgG fractions, after elution with 4.9 M MgCl2, reacted with CL. The antigen-binding characteristics of the IgG fractions purified from normal human serum (NHS) were similar to those of IgG fractions purified from sera of four patients with the anti-phospholipid syndrome (APLS). Competition assay confirmed the specificity of the binding of the purified IgG fractions to CL. The same results have been achieved with IgG fractions purified on Sepharose Protein-A column. The binding to CL was completely inhibited by either whole NHS and sera from various animal species, or by beta 2-glycoprotein I (beta 2-GPI). Our results support the notion of the existence of both natural anti-CL antibodies and serum inhibitor(s) in sera of healthy individuals. It is conceivable that in part the pathogenesis of APLS entails defects in the natural inhibitors of aCL antibodies.