In this study, the distribution and reisolation of Mycoplasma pneumoniae(Mp) were observed from the various tissues of BALB/c mice which were intraperitoneally pre-inoculated with Mp. In addition, the effect of Mp on the growth, phagocytic activities and nitric oxide production of microglial cells were also examined. The results were as follows; 1) Mp was reisolated from the various tissues such as lymph node, spleen, liver, kidney, brain and blood from one hour through 48 hours after intra-peritoneal inoculation of Mp in mice by the cultural method. Furthermore, it could also be confirmed from those tissues up to 72 hours by the indirect immunofluorescent antibody method. 2) There was no difference in the phagocytic activities between the control microglial cells and Mp stimulated microglial cells. 3) The growth of microglial cells in the medium was significantly increased by the stimulation with Mp compared with that of the control. 4) Nitric oxide production of mouse microglial cells was increased by the combined treatment if IFN-r and LPS or IFN-r and Mp or IFN-r, LPS and Mp, whereas, no increase was observed by either LPS or Mp alone. 5) Nitric oxide production of microglial cells primed with IFN-r was closely related with the dose of LPS and Mp in the dose dependent manner rather than that of the IFN-r. These results suggest that; i) Mp spreads to the various tissues of mice within one hour after intraperitoneal inoculation, ii) the growth of microglial cells increases by the infection of Mp, iii) microglial cells have phagocytic activities to C.albicans and iv) nitric oxide production of microglial cells was augmented by the infection of Mp. Increased nitric oxide production of microglial cells is regarded as an increase of the intracellular bactericidal activiteis of microglial cells. It is suggested, nonetheless, that the inflammatory response of the Mp infected tissues is augmented by the increase of nitric oxide.