Numerous studies on the cytokine production profile in Leishmania major infected susceptible and resistant mice have been carried out to elucidate the mechanisms of healing or non-healing of this infection. However, many methods may have failed to detect the actual cytokine production in the inflammatory foci. To overcome this problems, the ELISPOT assay was used to examine the spontaneous production of IL-4 and IFN-gamma in vitro by mononuclear cells from the spleen, lymph nodes and liver in L. major-infected susceptible BALB/c and resistant C57BL/6 mice. None of these mononuclear cells spontaneously produced IFN-gamma in either mouse strains in vitro in the absence of the corresponding antigen(s). However, liver mononuclear cells from infected BALB/c mice spontaneously produced IL-4 in vitro in as early as 2 weeks after the infection, but this was not observed in C57BL/6 mice. The IL-4 producing liver lymphocytes consisted of CD4+ and/or gammadelta+ T cells and uncharacterized cells. These results suggest that liver lymphocytes play some role in the establishment of Th2 prevalence in susceptible BALB/c mice, based on the importance of IL.4 production in the early phase of L. major infection in establishing Th2 dominance in this parasite susceptible mouse.