The effect of murine interleukin-12 (IL-12) on L-arginine-dependent biosynthesis of nitric oxide (NO) by mouse peritoneal cells was evaluated. Interleukin-12 was found to trigger considerably enhanced production of NO in a dose-dependent manner. Antibody neutralization studies indicated that the effect of IL-12 was mediated by IFN-gamma without apparent participation of TNF-alpha. Synergistic effects of IL-12 plus lipopolysaccharide (LPS) were also observed. Our data thus provide evidence that IL-12 is a powerful but indirect modulator of NO formation. These findings may contribute to the better understanding of various biologic effects of IL-12.