T lymphocytes from patients with visceral leishmaniasis treated in vitro with leishmania antigens are unable to proliferate and to produce gamma interferon. These patients have negative specific skin tests. Opposite results are obtained in patients with another clinical form of the disease named mucocutaneous leishmaniasis, in which both tests are positive. Nevertheless, patients with visceral leishmaniasis or mucocutaneous leishmaniasis, refractory to chemotherapy (antimonium complex), were cured when treated with antimonium in combination with gamma interferon, in spite of different immunological profiles. Different interpretative hypotheses of the reversion of chemoresistance induced by gamma interferon are discussed.