Laboratory tests are commonly used to aid in monitoring disease activity in patients with systemic lupus erythematosus. Typically, tests for antinative DNA, complement, acute phase reactants, and sometimes circulating immune complexes are used for this purpose. More recently, tests for complement activation products and immunocyte activation have been suggested. We measured antinative DNA, C4, Westergren sedimentation rate, iC3b, Bb, and sIL-2R levels in a group of 100 patients with SLE evaluated clinically for activity by a modified version of the Systemic Lupus Activity Measure (SLAM); iC3b and Bb were studied in 71 of the patients and sIL-2R in 52. We found that sIL-2R and sedimentation rate correlated best with SLAM, anti-DNA was intermediate, and C4, iC3b, and Bb correlated the least. None of the tests predicted severity of disease, but sIL-2R was highest in patients with active renal disease.