The present study deals with changes of lysosomal enzyme activity in cultured mouse pertioneal macrophages infected with .five species of mycobacieria. Infection with M. tuberculosis raised acid phosphatase activity in macrophages. M. bovis BCG and M. microti also caused rise in this enzyme activity but not β-glucuronidase activity. The cells infected with M. leprae showed decrease in both acid phosphatase andβ-glucuronidase activities significantly. No change was observed in activity of acid phosphatase in the cells infected with M. lepraemurium. Infection with this microbe caused no change or decrease in both β-glucuronidase and N-Ac-β-glucosaminidase activities not only within the cells but also in the culture media. The pattern of enzymatic response was of similar quality between the in vivo and in vitro grown M. lepraemurium. Discussion was made about the difference in enzymatic response between the cells infected with tubercle bacilli and the cells with leprosy bacilli.