Human endometrium had potent peroxidase activity and an antimicrobial effect. Relationships which existed between the human endometrial peroxidase and antimicrobial effect were studied. Human endometrial peroxidase activity was measured by the modified method of Himmelhoch. In the normal menstrual cycle, the peroxidase activity in the secretory phase was higher than in the proliferative phase. The peroxidase activity in endometrial carcinoma was remarkably higher than in the normal menstrual cycle. The organisms (E.coli, S.aureus) and peroxidase extract were incubated for 60 min at 37 degrees C. Bacterial viability was determined by the plate culture method. The viable cell count was decreased. Endometrial peroxidase had an antimicrobial effect on E.coli and S.aureus. But the antimicrobial effect on clinically isolated S.aureus exerted by the peroxidase was not effective. Although the peroxidase activity in the endometrial carcinoma was higher than in the normal endometrium, there was no detectable antimicrobial effect. Peroxidase of endometrial carcinoma was suspected of having some different characteristics from normal endometrium.