Interleukin-4 (IL-4), a pleiotropic cytokine, is a major regulator of the immune system and is considered crucial for the development of T helper cell type 2 (TH2) responses. The susceptibility of BALB/c mice to infection with Leishmania major has been associated with a polarized TH2 response and an inability to down-modulate IL-4 production. The role of IL-4 in vivo was examined directly by disrupting the IL-4 gene in BALB/c embryonic stem cells. Despite the absence of IL-4, the genetically pure BALB/c mutant mice remained susceptible to L. major infection, showed no signs of lesion healing or parasite clearance, and did not switch to a TH1 phenotype.