The circulating immune complexes have been detected in the sera of albino rats infected withPlasmodium berghei and rhesus monkeys infected with P.knowlesi by (i) quantitative cryoprecipitation assay and (ii) polyethylene glycol assay. In the rodent model, the levels of circulating immune complexes increased during infection and decreased considerably in the post-infection period. In the simian system, high levels were detected during peak parasitaemia. Polyethylene glycol precipitate obtained from the sera during acuteP. knowlesi infection when analysed by Immunoelectrophoresis was found to contain (i) monkey IgG, (ii) four other components of monkey plasma, (iii) two components of normal monkey erythrocytes and (iv) antigen(s) ofP. knowlesi.