We studied and characterized anti-bovine beta 2-glycoprotein I antibodies (aB beta 2-GPI) in sera from patients with antiphospholipid syndrome (APS) by ELISA. Bovine beta 2-glycoprotein I (beta 2-GPI) was purified by heparin affinity and DEAE ion-exchange chromatography, and identified on immunoblots using a monoclonal antibody against human beta 2-GPI and by amino acid sequence analysis. aB beta 2-GPI levels in the sera from 36 APS patients were measured by ELISA using purified bovine beta 2-GPI as an antigen. The mean +/- standard deviation level of aB beta 2-GPI was 17.4 +/- 22.0 units in the 58% of APS patients who were positive. There was a significant correlation (P = 0.003) between aB beta 2-GPI and anticardiolipin antibody (aCL) levels. aB beta 2-GPI from the sera of patients with APS was inhibited by bovine beta 2-GPI itself. Purified IgG from the sera of patients with APS showed that bovine beta 2-GPI was capable of acting as a cofactor for aCL. Purified bovine beta 2-GPI was useful antigen for conventional ELISA. aB beta 2-GPI may contribute to the further development of aCL analysis and to the understanding of the pathogenesis of APS.