Successful immunization of highly susceptible BALB/c mice against progressive infection by Leishmania mexicana amazonensis, using whole solubilized promastigotes was achieved. The best immunization schedule consisted of three weekly injections of 5 X 10(7) parasite equivalents. Intravenous was superior to intraperitoneal or subcutaneous immunization. Protection persisted for up to 2 months after immunization, and beneficial effects could be observed in long-term follow-up (24 weeks after infection). Immunized mice exhibited marked reduction in primary lesion size, as well as reduction of the number of parasites in the spleen, and developed less metastases. High titres of specific anti-L. m. amazonensis IgG antibodies resulted from immunization, but titres did not correlate with protection. Groups with widely differing pre-infection antibody titres were equally protected, and similar antibody titres resulted in different levels of protection. Immunization alone did not induce significant serum interferon-gamma levels and specific delayed-type hypersensitivity (DTH) reactions, but resulted in the persistence of positive (DTH) reactions after infection, at a time when infected control animals had suppressed responses. Resistance to leishmaniasis appears to depend on cell mediated immune mechanisms, and the possibility of immunization with a solubilized antigen without adjuvant is intriguing and opens new perspectives in this area.