Circulating immune complexes (CIC) were investigated by the [125I]Clq binding test, the complement fixation test (CFT) and optical density measurement after redissolving 3% polyethylene glycol precipitates of serum from patients infected by Schistosoma mansoni. A highly significant correlation was obtained among these three techniques. More than 60% of the patients demonstrated significantly higher values than control individuals. The level of CIC was found to be higher in the mild than in the hepatosplenic form of the disease. Parasite antigen, IgG, IgM and IgE were characterized in these CIC. In experimental schistosomiasis in mice, maximum levels of CIC, evaluated by the CFT, were observed between the 40th and the 70th day of infection.