The phagocytic and candidacidal activities of the peritoneal cells of Candida albicans-infected mice were studied 20 days following experimental infection. Both activities were enhanced during infection. The production of nitric oxide (NO) by the peritoneal cells of infected mice was determined, and an increase in the nitrite concentration in supernatants of peritoneal cell cultures was detected. The period of NO production by the peritoneal cells coincided partially with the period of enhanced C. albicans killing. The inhibition of NO synthesis by N-monomethyl-L-arginine was concomitant with inhibition of candidacidal activity. We conclude that NO synthesis is the primary candidacidal mechanism of the murine peritoneal cells activated by C. albicans infection.