Leishmaniasis is caused by the protozoan parasite Leishmania spp. In murine leishmaniasis, a T helper cell type-I (Th1) response, characterized by the secretion of interferon (IFN)-γ is necessary for clearing the infection, whereas a Th2 response, accompanied by the production of interleukin (IL)-5, can exacerbate the disease. Moreover, the early cytokine milieu is thought to play an important role in determining the outcome of infection. In human leishmaniasis little is known about this early cytokine response. Because of this, we cocultured human peripheral blood mononuclear cells (PBMC) with Leishmania major in vitro and measured the production of IFN-γ, IL-5, and IL-10. We also treated PBMC cultures with various cytokines and neutralizing anticytokines. We found that the principal cytokine produced was IFN-γ and that its production was regulated by IL-10 and IL-12. In contrast, only low levels of Th2 cytokines such as IL-5 were produced. Therefore, the Th1–Th2 dichotomy that exists in inbred strains of mice does not appear to apply to the response of humans to L. major. Rather, Th2 cytokines may play a role in regulating IFN-γ production.