The molecular size of circulating immune complexes in patients with systemic lupus erythematosus was determined by the C1q solid-phase assay after the sera were fractionated by sucrose-gradient ultracentrifugation. Circulating immune complexes in patients with membranous glomerulonephritis were uniformly large, sedimenting exclusively above 19S, whereas the immune complexes in patients with cerebritis were small, at or just above 7S. In lupus patients with diffuse proliferative glomerulonephritis and patients without renal involvement, immune complexes of both large and small sizes were found. Of patients without renal involvement, more circulating immune complexes were associated with active disease (n= 22, prevalence= 82%, mean level= 24 standard deviations) than with inactive disease (n= 17, prevalence= 41%, mean level= 41%, mean level= 6. 5 standard deviations). In patients with clinical evidence for renal involvement, circulating immune complexes were detected in all of five patients with membranous glomerulonephritis, in 88% of 17 patients with diffuse proliferative glomerulonephritis and in one of four patients with mesangial nephritis. Thus, in addition to the finding of an overall positive correlation between disease activity and circulating immune complex levels, circulating immune complexes of certain general molecular size ranges appear to be associated with different clinical manifestations of systemic lupus erythematosus.