The resolution of infections with the protozoan parasite Leishmania major in mice requires a Th1 response that is closely associated with the expression of IL-12, IFN-γ, and inducible NO synthase. Previous Ab neutralization studies or the use of mice deficient for both TNF receptors suggested that TNF plays only a limited role in the control of parasite replication in vivo. In this study we demonstrate that resistant C57BL/6 (B6. WT) mice locally infected with L. major rapidly succumb to progressive visceral leishmaniasis after deletion of the TNF gene by homologous recombination. A reduction of the parasite inoculum to 3000 promastigotes did not prevent the fatal outcome of the disease. An influence of the altered morphology of secondary lymphoid organs in C57BL/6-TNF−/−(B6. TNF−/−) mice on the course of disease could be excluded by the generation of reciprocal bone marrow chimeras. Although infected B6. TNF−/− mice mounted an L. major-specific IFN-γ response and expressed IL-12, the onset of the immune reaction was delayed. After in vitro stimulation, B6. TNF−/− inflammatory macrophages released 10-fold less NO in response to IFN-γ than B6. WT cells. However, in the presence of a costimulus, eg, L. major infection or LPS, the production of NO by B6. WT and B6. TNF−/− macrophages was comparable. In vivo, inducible NO synthase protein was readily detectable in skin lesions and draining lymph nodes of B6. TNF−/− mice, but its expression was more disperse and less focal in the absence of TNF. These are the first data to demonstrate that TNF is essential for the in vivo control of L. major.