The degree of resistance to a local Leishmania amazonensis challenge has been compared in lines of mice obtained by selective breeding for high or low immunoresponsiveness: High and Low antibody responder mice of Selections I and II (HI, HII and LI, LII lines) and high and low responder mice to T mitogen PHA (Hi/PHA and Lo/PHA). The aim of this preliminary study was to focus attention on genetic differences related with well defined immune characteristics. Clear-cut results were obtained, both HI and HII mice developed large and disseminating lesions, the rate of symptom aggravation being faster in HII, while LI and LII proved resistant to parasites, only small and transient lesions being observed for them during a 150 days follow up. The outcome of infection also differs in Hi/PHA and Lo/PHA mice, Hi/PHA having a resistant and Lo/PHA a susceptible phenotype.