OBJECTIVE To determine if IgM and IgA anticardiolipin (aCL) antibodies require beta 2 glycoprotein I (beta 2-GPI) as a cofactor for antibody binding. METHODS Sera were selected from 7 patients with systemic lupus erythematosus (SLE), 6 of whom had high IgM and 6 high IgA aCL antibody binding. Control sera were obtained from 2 healthy individuals with no aCL antibodies. Serum proteins were initially separated by sepharose CL6B get filtration chromatography, and IgM was further purified by affinity chromatography with mannan binding protein. IgA was isolated from CL6B filtrate by jacalin lectin affinity chromatography. Levels of beta 2-GPI in the immunoglobulin preparations were determined by antigen capture ELISA: Anticardiolipin antibody binding IgM and IgA was examined by ELISA with and without the addition of beta 2-GPI (10 micrograms/ml) or 4% normal human serum and expressed in optical density units (OD). RESULTS beta 2-GPI was required as a cofactor for IgM aCL antibody binding in 4 to 6 patients with SLE. In these, antibody binding to cardiolipin increased from (mean +/- SEM) 0.10 +/- 0.01 TO 1.06 +/- 0.22 (p = 0.005) with the addition of beta 2-GPI. For IgA, 5 of 6 patients with SLE demonstrated a requirement of beta 2-GPI as a cofactor. Antibody binding increased from 0.27 +/- 0.05 to 1.77 +/- 0.35 (p = 0.003) with the addition of beta 2-GPI. CONCLUSIONS beta 2-GPI is required as a cofactor for IgM and IgA aCL antibody binding.