T. cruzi clones and inbred mice are being used to develop experimental models for Chagas' disease. These studies demonstrate that a myocardiotropic clone of T. cruzi induces in C3H mice a pattern of myocarditis similar to human chagasic myocarditis. The possibility of inducing myocardial inflammation and fibrosis by spleen cell transfer from mice chronically infected with a myocardiotropic T. cruzi clone to uninfected normal mice has been investigated. In contrast with other reports, preliminary data showed that, myocarditis could not be induced by spleen cell transfer even with irradiated recipient mice. The immunization of inbred mice with low virulence T. cruzi clones does not protect against challenge with more virulent clones of the parasite. The infection of rats with two T. cruzi clones that differ in their pathogenicity for inbred mice was studied. The patterns of infection of rats with the two clones were similar to that occurring in C3H mice when infected with the same T. cruzi clones. The data indicate that the outcome of infection is greatly influenced by the genetic characteristics of the parasite stock.