The IgG subclass and light chain distribution of antiphospholipid antibodies (aPL) occurring in autoimmune patients were determined by means of two radioimmunoassays using either cardiolipin- or beta 2 glycoprotein 1 (beta 2GP1)-coated microtitre plates and mouse MoAbs. Of 50 sera selected for positivity of anticardiolipin antibodies (ACA) of the IgG isotype, 32 (64%) possessed anti-beta 2GP1 antibodies and their presence was closely associated with clinical features of the antiphospholipid syndrome. Good correlations were found between ACA and anti-beta 2GP1 antibodies when considering antibody level and patterns of light chain and IgG subclass, suggesting that, overall, the same antibodies were being measured. Light chain analysis showed the polyclonal origin of these antibodies and, in most sera, a trend towards use of lambda chain. Among sera positive for anti-beta 2GP1 antibodies, IgG2 was the major subclass reactive with beta 2GP1 and cardiolipin (87% and 74% of the IgG antibody activity, respectively). In contrast, in the group of 18 sera lacking anti-beta 2GP1 antibodies, ACA were largely restricted to IgG3, with a lesser contribution by IgG1. A few selected sera from the anti-beta 2GP1-positive group were shown to contain mixtures of antibodies that required beta 2GP1 (restricted to IgG2 present in large amounts) and did not require this cofactor (restricted to IgG3 and/or IgG1 present in low amounts) for their reactivity with cardiolipin. There was no contribution of glycosylation to the epitopes recognized by anti-beta 2GP1 antibodies, even though human anti-carbohydrate antibodies are restricted to the IgG2 subclass. These findings further emphasize the intra- and interindividual heterogeneity of aPL, and should help to discriminate clinically relevant specificies.