Evidence has been presented suggesting that circulating immune complexes occur in over half of the sera of patients with rheumatoid arthritis. These IgG containing complexes are small, eluting between IgG and IgM on gel filtration on Sepharose 6B and are not seen in the sera of healthy controls. These complexes were detected in the sera of both seronegative and seropositive patients and their quantity did not correlate with IgM rheumatoid factor titre. The quantity of complexed IgG was estimated from a ratio derived from the IgG profile obtained by gel filtration of the serum. This quantity correlated significantly with the degree of inhibition by the rheumatoid sera of cytolysis in vitro of IgG sensitized target cells by K cells from human peripheral blood. A significant inverse correlation was observed between the quantity of serum complexes and the chemotactic index of the circulating polymorphonuclear leucocytes obtained from the same rheumatoid patient. It is suggested that ingestion of these complexes may be implicated in the reduction in chemotaxis observed in patients with rheumatoid arthritis. There was no correlation between the quantity of the complexes in the sera and other clinical, haematological, and biochemical measurements.