The heavy metal lead is an environmental toxicant that can compromise host defense systems and induce pathophysiological changes in many organ systems. We report herein that low levels of lead can efficiently block nitric oxide production in vitro by murine splenic macrophages, which could contribute to lead's inhibition of pathogen killing by macrophages. We observed that lead was able to suppress nitric oxide production by concanavalin A-stimulated murine splenocyte cultures. Since splenocyte cultures contain macrophages, lymphocytes, and other cell types, we further delineated the target(s) of the inhibition. High- and low-dose interferon-gamma and tumor necrosis factor-alpha-induced nitric oxide production by macrophages was readily blocked by low levels of lead (ID50 = 0.35-0.95 microM or 0.07-0.19 ppm); however, activated T cell membrane-enhanced nitric oxide production by macrophages was less sensitive to inhibition by lead. Since lead has been reported to preferentially enhance the activation of T helper-2 cells, which produce interleukin-4, a regulator of macrophage activities, the role of interleukin-4 in the inhibition by lead also was assessed. Interleukin-4 did not mediate the inhibition; instead, interleukin-4 appeared to be an enhancing factor for nitric oxide production in vitro. The observed effects of lead on nitric oxide production may provide an explanation for some of the lead-induced abnormalities associated with host defense and organ systems in addition to the immune system.