Seventy-one patients with systemic lupus erythematosus (SLE), seen in an outpatient setting for follow-up evaluation during a 3-mo period, were tested (in addition to routine lupus monitoring studies) with enzyme immunoassays (EIAs) for anti-DNA, iC3b, and factor Bb to determine the relationship of these test results to disease activity. SLE activity was scored by four previously reported scoring systems, and six patients were identified as active by all four systems. We found that the EIA for anti-DNA was the best indicator of disease activity and that iC3b and Bb were not informative for this purpose in this group of patients. Mean iC3b levels were higher in a subset of seven patients with past biopsy evidence of severe (WHO class IV) glomerulonephritis than in the rest of the study population, even though none of these patients had active disease at the time of this study.