The production and roles of endogenous interleukin-4 (IL-4) and IL-10 in a sublethal infection with Listeria monocytogenes were studies in normal mice and anti-gamma interferon (IFN-gamma) monoclonal antibody (MAb)-pretreated mice. In normal mice, the expression of mRNAs for IL-4 and IL-10, which was amplified by reverse transcription-PCR, was induced in the spleens and livers either early or late in infection, although the serum IL-4 and IL-10 were not detectable by enzyme-linked immunosorbent assays. In vivo administration of anti-IL-4 MAb showed no effect on antilisterial resistance, whereas anti-IL-10 MAb partially diminished the defense. In anti-IFN-gamma MAb-pretreated mice, a delay in the bacterial elimination from the spleens and livers was observed and high titers of serum IL-4 and IL-10 were induced late in infection. Production of endogenous IL-4 and IL-10 was suppressed in both CD4+ cell-and CD8+ cell depleted mice. The suppression of antilisterial resistance in anti-IFN-gamma MAb-pretreated mice was canceled when anti-IL-4 MAb was injected late in infection, whereas anti-IL-10 MAb showed no effect. These results suggest that the host immune responses were polarized into the T-helper 2 phenotype in anti-IFN-gamma MAb-pretreated mice and that inhibition of host resistance against L. monocytogenes by depletion of IFN-gamma might be attributable to IL-4 produced by T cells polarized into the T-helper 2 phenotype as well as the inhibition of the IFN-gamma effects.