Leishmania spp. are intracellular protozoan parasites that invade and replicate within macrophages. In a previous report, we have demonstrated that the growth of intracellular amastigotes could be controlled by inhibition of arginase. This enzyme, induced in host cells by Th2 cytokines, synthesizes L-ornithine which can be used by parasites to generate polyamines and proliferate. In this study, we have designed experiments to better analyse the dependence of parasite proliferation on arginase induction in infected macrophages. Treatment of Leishmania major-infected BALB/c macrophages with interleukin (IL)-4, IL-10 or transforming growth factor-β, which are all inducers of arginase I in murine macrophages, led to a proportional increase in the number of intracellular amastigotes. Moreover, parasite proliferation and arginase activity levels in macrophages from the susceptible BALB/c mice were significantly higher than those from infected C57BL/6 cells when treated with identical doses of these cytokines, indicating that a strong correlation exist between the permissibility of host cells to L. major infection and the induction of arginase I in macrophages. Specific inhibition of arginase by Nω-hydroxy-nor-L-arginine (nor-LOHA) reverted growth, while L-ornithine and putrescine promoted parasite proliferation, indicating that the parasite cell division depends critically on the level of L-ornithine available in the host. Therefore, arginase induction in the context of a Th2 predominant response might be a contributor to susceptibility in leishmaniasis.