The protective effects of immunization with an antigen derived from epimastigote forms of Trypanosoma cruzi against challenges with vector metacyclic or bloodstream forms of the parasite were investigated. A marked degree of protection was observed in the immunized mice after challenged with bloodstream or insect-transmissible metacyclic forms. High rates of survival in the immunized groups were accompanied by relatively low, short-lasting parasitemias in some of the animals and significant percentages of immunized mice never developed a measurable parasitemia during the course of the experiments. In contrast, all of the non-immunized animals showed high levels of parasitemias. Similar results were obtained whether the metacyclic challenge was by the intraperitoneal or by the ocular route for which conditions mimicking a natural infection were selected. These results emphasize the protection that immunization confers against challenge with insect-transmissible forms of T. cruzi, and the feasibility of protecting a highly susceptible host against an otherwise lethal acute infection similar to one occurring naturally.