Mice with a genetically engineered deficiency for either IL-4 or IFN-gamma R1 (single mutants), and IL-4/IFN-gamma R1 (double mutants) on the Balb/c and 129Sv background were used to study the course of infection with Leishmania major. In contrast to genetically resistant 129Sv wildtype mice, IL-4/IFN-gamma R1 double mutant mice developed fetal disease with parasite dissemination to visceral organs similar to mice lacking IFN-gamma R1 only. Balb/c mice, which are exquisitely susceptible to L. major, were rendered resistant to infection by disruption of the IL-4 gene. As compared to homozygous IL-4+/- mice, heterozygous IL-4+/- mice, heterozygous IL-4+/- animals consistently developed smaller lesions with less ulceration and necrosis, indicating the likelihood of gene-dosage effects. This implicates that the magnitude of the IL-4 response determines the severity of disease. CD4+ T cells of IL-4-deficient mice showed impaired Th2 cell development, as assessed by quantitative RT-PCR of characteristic cytokines. Development of resistance is not explained by default Th1 development, because this was observed only at very late stages of infection. Moreover, the induction of inflammatory cytokines (e.g., IL-1 alpha, IL-1 beta, TNF-alpha, IL-12) together with iNOS in the lesion and draining lymph nodes was not altered in the absence of IL-4.