Delayed-type skin hypersensitivity in rabbits infected with homologous or heterologous T. cruzi has been elicited upon injection of 50 micrograms of protein in the T12E antigen derived from a parasite clone. The specificity of this reaction was indicated by absence of skin reactivity in control rabbits that received same quantity of antigen. Also, the intradermal injection of the antigen in five occasions in control rabbits neither induced seroconversion nor shifted their skin sensitiveness. On the other hand, chagasic rabbits that underwent a series of five skin testings one week apart, did not alter the intensity of the skin reactivity, and their specific serum antibody titers remained similarly high. Of interest, the ECG patterns remained unchanged after a series of five skin testings in control and in chagasic rabbits. This study shows that the T12E antigen might be an useful tool for diagnosing T. cruzi infections, besides serving as an immunologic marker for morbidity in Chagas' disease.