Autoimmune antiphospholipid antibodies are a hallmark of patients with antiphospholipid syndrome, and require a protein cofactor, beta 2-glycoprotein I, to bind anionic phospholipids. In these same patients, moreover, IgG directly binding beta 2-glycoprotein I are described. We found high plasma titres of both IgM and IgG anti beta 2-glycoprotein I antibodies in a patient with catastrophic antiphospholipid syndrome. After passing plasma through a Sephacryl S-300 column, an identical distribution pattern between anti beta 2- glycoprotein I and anticardiolipin antibodies was observed. Moreover, when IgG immunocomplexes were isolated from a high molecular fraction, IgM anti- beta 2-glycoprotein I and anticardiolipin antibodies were detected. Thus IgG, IgM ad IgG-IgM complexes of anti-beta 2-glycoprotein I antibodies are present at the same time in a patient with antiphospholipid syndrome.