Several studies have been reported that nitric oxide(NO) produced by macrophage shows antimicrobial activites, antitumor effects, and host cell damages. In order to understand the NO-induced host cell damage in the inflammatory reaction by M. pneumonia infection, we measured the NO production in the mouse macrophage and mouse exposed to M.pneumoniea antigen(MPA) or mitogens such as lipopolysaccharide(LPS), concanavalin A(Con A), phytohemagglutinin P(PHA-P), and phorbol 12-myristate 13-acetate(PMA). As a measure of NO level, NO2-concentation was determined colorimetrically using the Grriess reaction. One hundred μl of sample was mixed with 100μl of Griess reagent in the ELISA plate, and the absorbance of mixture was measured at 540 mm with a Titert multiscan pllate reader MCC 340. The results were as follows : 1) The NO production was significantly increased by M.pneumoniea antigen or mitogens(LPS, Con A, PHA-P, PMA) in the spleni macrophage cultivated in vitro, the trend beign more pronounced in longer cultivation. 2) When the macrophage was exposed to a high concenntration of MPA and a mitogen, the NO production was increased with LPS and decreased with PHA-P and PMA, as compared with antigen alone. 3) The NO concentration in the sera of mice was increased by the injection of MPA or LPS, however the NO concentration by PHA-P, Con A, or PMA was similar to or decreased as compared with the control group. In mice exposed to both MPA and a mitogen, the NO concentration was lower than in those exposed to antigen alone. These results suggest that enhancement of NO production in the macrophage exposed to MPA may induce host cell damage in the inflammatory reaction. Following stimulation of macrophage with MPA, the NO production may be either augmented or suppressed according to the kind of mitogens and the concentration of MPA.