CNPq, LIM (Hospital das Clinicas) and Fundação Butantan. C. A.Serum levels of interferon-gamma (IFN-y) were evaluated in Calomys callosus and Swiss mice during the course of infection by four strains of Trypanosoma cruzi. All strains stimulated the production of this interleukine; however, the timing of its onset and permanence varied among strains and between the two animal models. When chronically infected animals with no detectable serum IFN-y were challenged with the homologous strain, they produced quantities comparable with those obtained during the acute phase of infection. In C. callosus there was a correlation between H202 liberation by peritoneal macrophages and serum IFN-y levels, whereas no such correlation was found in mice. C. callosus had a higher capacity to heal histopathological lesions, whereas lesions in mice were progressive, The results obtained suggest that C. callosus develops welladapted immune mechanisms that may be important for its role as a reservoir of T. cruzi.