Antiproliferative drugs (Azathioprine and Methotrexate) were able to enhance a T. cruzi virulent infection in mice, but failed to induce evident infection in animais previously vaccinated with the live PF strain of the same parasite. Vinblastine (Vbl) had a similar effect with the virulent Y strain, but promoted the appearance of positive blood eultures in animais vaecinated and treated previously with the drug. Negative results are obtained if the immunosupressive agent is used 15 days after the vaccine. The flagellates isolated from the vaccinated Vbl treated mice remained avirulent for baby mice, after i. p. inoculation. The immunity induced by the PF strain is probably sterile i. e. does not imply in the persistence of a latent infection (premunition).