Upon interferon-gamma (IFN-gamma) stimulation, murine macrophages (Mphi) produce nitric oxide (NO) through expression of inducible nitric oxide synthase (iNOS). Interleukin (IL)-4 treatment, even delayed 12 h relative to IFN-gamma, antagonized this induction, whereas infection with herpes simplex virus type 2 (HSV-2) or treatment with tumour necrosis factor-alpha exerted a synergistic effect, which partly compensated for the antagonistic effect of IL-4. Neither IL-4 nor HSV-2 affected the IFN-gamma-activated Jak-STAT (Janus kinase-signal transducer and activator of transcription) pathway or altered the levels of IFN-gamma-induced interferon regulatory factor (IRF)-1 expression, which is STAT1-dependent and known to play a central role in IFN-gamma-mediated gene induction. The effect of IL-4 was completely dependent on de novo protein synthesis, indicating that a direct activation of latent inhibitors is not sufficient to explain the inhibitory effect of IL-4. Furthermore, IL-4 substantially augmented the IFN-gamma-induced expression of IRF-2, which is known to compete with IRF-1 for the DNA recognition site, ISRE (interferon-stimulated response element). Our findings could indicate that IL-4 suppresses IFN-gamma-stimulated iNOS transcription by elevating the level of IRF-2 which, through competition, prevents IRF-1 from binding to ISRE in the iNOS promoter. The virus-induced effects on iNOS and NO levels in IFN-gamma-stimulated Mphi do not seem to involve the Jak/STAT pathway or a differential expression of IRF-1 and IRF-2.