To investigate the role of tumor necrosis factor (TNF) in protective immune responses to Mycobacterium tuberculosis and M. bovis Bacillus Calmette Guérin (BCG), we have used transgenic mice unable to use TNF because of the expression of high amounts of a soluble TNF receptor (R) type I (sTNFR1) fusion protein, and studied resistance of these mice to infection by lethality assays, evaluation of bacterial recovery and histologic examination. These mice showed a strongly increased sensitivity to M. tuberculosis and BCG infections, with bacterial overgrowth and marked inhibition of macrophage differentiation within granulomas; after M. tuberculosis infection, this resulted in extensive lesions of caseous necrosis in the lung. To explore the respective roles of TNF and interferon (IFN)‐γ in resistance to BCG and granuloma differentiation, controls and sTNFR1‐transgenic mice were compared to IFN‐γR mutant mice and mice double defective in TNF and IFN‐γ activity (obtained by crossing transgenic and mutant mice). The three groups of deficient mice showed a strongly enhanced susceptibility to BCG infection, with the following decreasing order of sensitivity between groups: TNF + IFN‐γ → TNF → IFN‐γ‐deficient mice. The hepatic granulomas of IFN‐γR mutant mice were small and contained eosinophils but few differentiated macrophages; compared to those of sTNFR1‐transgenic mice, acid‐fast bacilli were less numerous within the macrophages. Granulomas of double‐deficient mice were strikingly different by their very large size and cellular content, made up large numbers of polymorphonuclears, eosinophils, and cells undergoing apoptosis, but without detectable differentiated macrophages; acid‐fast bacilli were spread in the lesions. These studies show the essential role of both TNF and IFN‐γ in the development, during mycobacterial infections, of protective granulomas containing highly differentiated macrophages capable of destroying ingested bacteria, and emphasize that these two cytokines act synergistically in granuloma formation.