Materials with the Clq binding properties of soluble immune complexes (IC) were found in sera from 11 of 51 consecutive (22%) children with juvenile rheumatoid arthritis (JRA) and in 17 of 20 adults with active seropositive rheumatoid arthritis (RA). IC appeared more frequently in children with systemic onset disease whereas antinuclear antibody (ANA) was found more frequently in sera from those with pauciarticular disease. Only 3 JRA sera contained anti-immunoglobulin (rheumatoid factor); those 3 also had high Clq binding activities. Seven of 50 patients (14%) carried HLA-B27 but B27 was not associated with high Clq binding activity or presence of ANA. The presence of free ANA more frequently in children with mild disease and IC more frequently in children with relatively severe disease suggests that children with systemic JRA may have a relative defect in antibody-forming capacity or reticuloendothelial function which results in decreased clearance of circulating IC. Alternatively, systemic, polyarticular, and pauciarticular JRA may represent a spectrum of clinically similar diseases resulting from different etiologic agents.