In this work we studied the IgG isotypes induced in mice immunized with two Trypanosoma cruzi acidic antigenic fractions (F IV and Eas 4.5) and the level of protection to a later infection with parasites. F IV is a cytosolic antigen from epimastigotes, and Eas 4.5 is an exoantigen released by trypomastigotes. The most relevant epitopes of Eas 4.5 are carbohydrates. A high prevalence of IgG1, low levels of IgG3 and no IgG2 antibodies against F IV and Eas 4.5 were found in sera obtained 2 weeks after the last antigen dose from animals immunized with F IV (group I) or Eas 4.5 (group II). Immunized mice from both groups were infected with trypomastigotes, and the parasitemias detected later on were significantly lower than in control groups (p less than 0.01, group I; p less than 0.001, group II). The amount of IgG2-specific antibodies, which was only detected using epimastigotes as antigen in ELISA, was significantly increased after the infection, but no major changes were seen in the profiles of other isotypes.