To study sensitivity and specificity of antibodies to beta 2-glycoprotein I (B2GP-I) and antibodies to cardiolipin (CL) in diagnosis of antiphospholipid syndrome (APS), we examined 19 patients with primary antiphospholid syndrome (PAPS), 23 patients with secondary APS (SAPS) and systemic lupus erythematosus (SLE) and 73 patients with SLE. Antibodies to B2GP-I and CL of IgG isotype were measured at enzyme immunoassay. The levels of IgG aCL and IgG aB2GP-I in the serum of PAPS patients were 69.9 +/- 116.0 GPL, 74.1 +/- 117.4 SGU, respectively; of SAPS and SLE patients 60.5 +/- 68.9 GPL and 66.2 +/- 88.3 SGU, respectively. These values were significantly higher than in SLE patients (19.5 +/- 27.6 GPL, p = 0.0025 and p = 0.0012; 14.5 +/- 41.9 SGU, p = 0.0001, respectively) and donors (3.9 +/- 3.8 GPL, p = 0.0001; 5.7 +/- 1.2 SGU, p = 0.001). By IgG aCL and IgG-aB2GP-I, PAPS and SAPS patients differed insignificantly (p > 0.05), but these values correlated positively (r = 0.85; p < 0.0005 for PAPS and r = 0.9, p < 0.005 for SAPS). Simultaneous detection of IgG aCL and IgG aB2GP-I occurred in 36.8% in PAPS, 52.4% in SAPS and 12.3% in SLE. 10.5% PAPS patients were positive by IgG aCL as well as 9.5% with SAPS and 13.4% with SLE. Isolated rise of IgG aB2GP-I concentration was observed in 21.1% patients with PAPS, in 9.5% patients with SAPS and 9.6% patients with SLE. Of 43 patients with a history of thrombosis, 46.5% were positive by IgG aCL and IgG aB2GP-I, 7.0% positive only by IgG aCL and 11.6% only by IgG aB2GP-I. Levels of IgG aCL and IgG aB2GP-I in patients with thrombosis (64.7 +/- 87.5 GPL and 66.0 +/- 94.8 SGU, respectively) were significantly higher than in patients without them (19.6 +/- 3.8 GPL, p = 0.009 and 16.4 +/- 52.2 SGU, p = 0.0001). In venous thrombosis IgG aCL and IgG aB2GP-I were higher than in arterial thrombosis (p < 0.004). For diagnosis of APS sensitivity and specificity of IgG aB2GP-I and IgG aCL were 60.0 and 83.8%; 57.1 and 73.5% (p > 0.05), respectively. As to thrombosis, the sensitivity and specificity were 58.1 and 84.6%; 54.5 and 72.7% (p > 0.005), respectively. Thus, IgG aB2GP-I is an essential additional marker of APS. In the presence of APS symptoms, but in negative results of APS test it is justified to confirm APS diagnosis by aB2GP-I test results. To make APS diagnosis more accurate, it is valid to make simultaneous measurements of aCL using standard B2GP-I dependent enzyme immunoassay and aB2GP-I.