The relationship between the effect of chloroquine treatment on circulating immune complexes in patients with rheumatoid arthritis (RA) was determined by the 125I Clq binding assay. Three groups were studied: (a) 20 patients treated daily for 6 months with chloroquine sulphate 250 mg plus prednisone 7.5 mg and nonsteroidal antiinflammatory drugs (NSAID); (b) 20 patients not taking antimalarials but treated with prednisone and NSAID; (c) 41 controls. Results suggest that chloroquine sulphate therapy induced a marked decrease of immune complexes in patients with RA, an effect not observed with treatment by prednisone and NSAID only.