A protein purified from Leishmania donovani promastigotes, dp72, was shown to partially protect BALB/c mice against a challenge by this parasite. Immunized mice infected i.v. with 10(7) L. donovani promastigotes showed a 0, 60, and 78% reduction in liver parasite burden compared with the control mice at each time point examined after challenge, 1, 20, and 108 days, respectively. Western blotting demonstrated that the sera from the immune mice, which reacted specifically with dp72 in lysates of L. donovani, cross-reacted with one major band in total homogenates of Leishmania major and Leptomonas collosoma. Lymphocyte proliferation to crude and pure parasite Ag was also examined in mice immunized with dp72. Strong proliferation was found at low concentrations of crude L. donovani Ag (0.5 micrograms/ml) and with pure dp72. Proliferation at higher concentrations to crude L. major and L. collosoma Ag was observed. Little or no reaction (stimulation index < 1.0) was seen with other pure leishmanial Ag, including gp70-2, the promastigote surface protease, and lipophosphoglycan. Depletion in vitro of the CD4+ T cell subset from immune spleen cells abolished proliferation to dp72, whereas depletion of CD8+ T cells enhanced proliferation to the pure Ag. Experiments in vivo showed that immunized mice treated with antibodies to either CD4+, CD8+, both T cell subsets, or to IFN-gamma had larger LPB (178, 173, 176, and 130%, respectively) after challenge with L. donovani than nonimmunized controls. Mice immunized with dp72 but treated with either PBS or mouse Ig showed reduced LPB, 81 and 61%, respectively, compared with the nonimmunized animals. BALB/c mice immunized with dp72 were also protected against L. major which causes cutaneous leishmaniasis. Immunized mice infected with either 10(4) or 10(6) promastigotes did not develop lesions. Limiting dilution assays confirmed the protection.