Circulating schistosome antigens (CSA) and circulating immune complexes (CLC) were investigated in rats infected with Schistosoma mansoni. The radioimmunoprecipitation-polyethylene glycol (PEG) assay (RIPEGA), with 125I-labelled anti-S. mansoni anti-serum, detected CSA during two distinct periods of the infection; the first between the 11th and the 14th week of infection and the second between the 11th and 14th week after infection. The CH50 deviation test revealed the presence of CIC in sera from infected rats, approximately at the two periods when CSA were detected. At 6 weeks of infection, the levels of CIC in infected rats were not different from those in control rats. However, a more sensitive method characterized IgG2a in C1q-binding C1C from infected rats. At weeks 5 and 6, IgE immune complexes were also detected in the serum from infected rats. In fact, the use of RIPEGA on the material eluted from infected rat serum after passage through an anti-IgE immunosorbent showed the presence of schistosome antigen at week 4, and at higher levels at week 6. Levels of 50% haemolytic complement in infected rat serum were lowered between the 2nd and the 4th week, the 5th and the 8th week and after the 12th week of infection. The possible role played by CIC in the protective mechanisms to a S. mansoni challenge infection in rats is discussed.