The clinical significance of anti-beta 2 glycoprotein I (beta 2-GPI) antibodies was evaluated in patients with antiphospholipid syndrome (APS), primary and secondary to systemic lupus erythematosus (SLE). Anti-beta 2-GPI were tested in 120 patients (39 primary APS, 32 APS with SLE and 49 SLE without APS) by ELISA utilizing irradiated plates in the absence of cardiolipin. Anticardiolipin antibodies (aCL) and antiphosphatidylserine antibodies were also measured in the same patients using standardized assays. Anti-beta 2-GPI titres correlated strongly to those of aCL (r = 0.816, P = 0.0001), and to those of antiphosphatidylserine antibodies (r = 0.841, P = 0.0001). Anti-beta 2-GPI were detected in 53.5% of APS patients (38/71), but only in 4.1% of SLE patients without APS (2/49). In the latter group, 24.5% (12/49) of patients had a positive titre of aCL. The anti-beta 2-GPI assay showed higher specificity for APS than the aCL in APS (96 vs 75%, respectively, chi 2 = 6.75, P = 0.00094). Our findings suggest that the assay of anti-beta 2-GPI may improve the specificity for APS.