Grant AI-16282 (National Institutes of Health - USA) and from the Conselho Nacional de Desenvolvimento Cientifico e Tecnològico (CNPq-Brasil).We have infected BALB/c and C57BL/6 mice with a cloned Leishmania mexicana amazonensis population, obtained from the "Maria" strain. Progres¬ sion of infection and histopathological examination confirmed the extreme susceptibility of BALB/c mice and the resistant pattern ofthe C57 BL/6. Anti- Leishmania antibody titers were higher in BALB/c than in C57BL/6 mice through the period of infection. Tests of delayed type hypersensitivity reaction with Leishmania antigens were positive in both strains in the beginning ofthe infection, but were negative later on in BALB/c mice. Our results are similar to those obtained with mixed parasite populations, and rule out the possibility of selection among different parasite subpopulations as responsible for the diver¬ gent course of the disease exhibited by these two strains of mice.