Nitric oxide (NO) is a multifunctional mediator that is involved in a variety of pathologic and physiologic processes. Few studies have addressed the effect of lactic acid bacteria (LAB), especially Lactococcus lactis strains used in dairy products, on inducible nitric oxide synthase (iNOS) induction as a component of the host's gastrointestinal immune response. We investigated the ability of L. lactis strains to induce NO synthesis in the murine macrophage-like cell line J774.1 and in peritoneal macrophages from mice. The degree of NO induction was specific to the L. lactis strain used. Compared with the no-treatment control, heat treatment of L. lactis cells decreased NO and TNF-α levels but further stimulated interleukin (IL)-12 production. Adding L. lactis cells to peritoneal macrophages dose-dependently increased the production of NO and IL-10 but decreased that of IL-12p70. Adding L. lactis cells to interferon-γ-stimulated J774.1 cells enhanced cell death and the production of NO and IL-12p40, whereas addition of 1400W, a specific inhibitor of iNOS, decreased NO production and cell death. Conversely, adding 1400W to J774.1 cells further enhanced IL-12p40 production, suggesting that IL-12 production is perturbed by excess endogenous NO. IL-12 production is thought to be a marker of improved immunostimulation. Our results suggest that IL-12 production could be increased by limiting endogenous NO production.