To investigate the role of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) in the resistance to Paracoccidioides brasiliensis (Pb) infection, mice with homologous disruption of the IFN-gamma (GKO) or TNF-alpha receptor p55 (p55KO) were infected with the parasite. GKO and p55KO, but not wild-type (WT) mice, were unable to control the growth of yeast cells and the mice succumbed to infection by days 16 and 90 after infection, respectively. Typical inflammatory granulomas were found only in WT mice. In contrast, knockout mice presented an inflammatory infiltrate composed of a few neutrophils, mononuclear, epithelioid, and multinuclear giant cells forming incipient granulomas in GKO mice and without granuloma formation in p55KO mice. Besides, both groups of knockout mice exhibited elevated numbers of yeast forms in agreement with colony-forming unit counts in organs. Compared with WT, splenocytes from infected GKO mice cultured with the Pb F1 fraction produced lower TNF-alpha levels, whereas leukocytes from infected p55KO mice produced similar amounts of TNF-alpha but higher levels of IFN-gamma. Moreover, splenocytes from infected WT mice produced higher levels of nitric oxide (NO) resulting in a lower T-cell proliferative response to Con A than uninfected WT, or infected p55KO and GKO mice. On the contrary, the addition of IFN-gamma to splenocytes from infected GKO mice resulted in higher NO production and lower T cell proliferation. Taken together, these findings suggests that endogenous TNF-alpha, acting through the p55 receptor, and IFN-gamma mediate resistance to Pb infection and induce NO production that determines marked T cell unresponsiveness.