CD154, a member of the tumor necrosis factor receptor family, is involved in several biological responses. In the sera of systemic lupus erythematosus (SLE) patients, the levels of sCD154 have been shown to be increased, however, few reports have dealt with the biologically active tetramer. Here, we assessed the biological activity of the serum CD154 tetramer using bioassays for BC activation and production nitrite or peroxide. The patients showed a markedly increased total sCD154 serum concentration (12.5 +/- 8.2 vs. 3.9 +/- 1.2 ng/ml; p < 0.001). ba-sCD154 was significantly increased in non-treated patients (7.4 +/- 3.4 ng/ml, n = 22; p < 0.001) and patients with the highest SLE disease activity index (SLEDAI) scores (5.3 +/- 2.9 ng/ml, n = 8), but not in stable patients (1.3 +/- 1.2 ng/ml, n = 30) whose values were similar to normal healthy donors (NHD; 0.8 +/- 0.2 ng/ml). Patients with SLEDAI above 8 that recovered after successful treatment displayed significantly decreased levels of ba-sCD154. We conclude that the bioassay is a useful tool discriminating active and stable SLE, as well as non-treated patients.