Gel filtration through Sephadex G-200 was used to detect anticomplement activity of sera from patients with Schistosoma mansoni infections or Chagas' disease. Protein content, immunoglobulins and hemolysis-inhibiting activity were assessed in the chromatographic effluent. The proteins were distributed as usual among three regular peaks. The presence of IgM (first peak) and IgG (second peak) in the fractions was demonstrated by the Ouchterlony method. No immunoglobulins were detectable in the third peak. A heat-labile activity which abolished the lysis of sensitized sheep red blood cells in the presence of guinea pig serum was eluted from the column in the ascending and top region of the first protein peak and in the top region of the second protein peak (fractions from schistosomiasis or chagasic patients). Furthermore, a potent anti-complement activity was eluted from sera of chagasic patients far from the first and second protein peaks, just before the third peak.