OBJECTIVE To study the relationship of serum IgG anti-F(ab')2 and clinical disease activity in 108 patients with systemic lupus erythematosus (SLE) and to determine whether low serum anti-F(ab')2 with active renal disease is accompanied by deposition of anti-F(ab')2 in renal immune complex lesions. METHODS We studied 108 patients with definite SLE over a 5 yr period and assayed serum anti-F(ab')2 levels in relation to degree of clinical disease activity. Renal biopsy eluates of 26 patients with SLE were examined by enzyme linked immunosorbent assay (ELISA) for relative amounts of IgG, anti-DNA, and IgG anti-F(ab')2. RESULTS Active SLE was strongly associated with low serum anti-F(ab')2. SLE renal biopsy eluates frequently contained high levels of IgG and IgG anti-DNA and lower, but definite, IgG anti-F(ab')2 activity. When specific activity of IgG anti-DNA and IgG anti-F(ab')2 was compared between kidney biopsy eluates and concomitant serum, marked relative renal concentration was found for both anti-DNA (19-fold) and anti-F(ab')2 (74-fold). Some biopsy eluates contained IgG antibodies bearing apparent double specificity for both DNA and F(ab')2. CONCLUSION Active SLE is often associated with low serum anti-F(ab')2. Relative enrichment over specific activity in serum of both IgG anti-F(ab')2 and anti-DNA in SLE kidney biopsy eluates may indicate participation of both reactants in the glomerular disease process. Low serum anti-F(ab')2 in active SLE may reflect down modulation of failure of idiotypic control mechanisms associated with disease progression.