The heating for 30 minutes at 56°C not only modifies the electrophoretic profiles of the Chagasic sera, but also their specific reactivity in complement-fixation with Trypanosoma cruzi antigens. the complement fixation titers are lower in inactivated sera and the avidity for complement of the immune-complex is reduced. It was found a correlation between the diminution of the CF titers and the reduction of the avidity for complement. The effects of inactivation on the specific reactivity of Chagasic sera was not observed with immunofluorescent antibody test or with hemagglutination. This finding does no support the hypothesis of thermolability of chagasic antibodies.