Results of the solid phase F(ab')2 anti-C3 radio-immunoassay for immune complexes were abnormal in sera from some patients with juvenile arthritis and in many patients with other forms of chronic inflammation. However, C3 failed to block positive reactions of sera from patients even though it did block the reaction when aggregated IgG was added to normal sera. Moreover, positive sera reacted with nonspecific F(ab')2 fragments from several species. High correlation coefficients were obtained between abnormal results in the anti-C3 assay with juvenile arthritis sera and reactions with goat, mouse, and rabbit F(ab')2, but less so with human F(ab')2. Sera from patients with systemic lupus erythematosus or cystic fibrosis had similar reactions but different species specificities. After sucrose density fractionation, the reactivity with nonspecific anti-F(ab')2 was only in the IgG-containing fractions, but anti-C3 activity was in both IgG-containing fractions and the fractions between IgG and IgM. We conclude that anti-F(ab')2 antibodies are common in children with inflammation and may interfere with interpretation of assays using F(ab')2 anti-C3 for detection of complexes.