Antibodies to Propionibacterium acnes in patients with tumor, leprosy, schizophrenia and normal human were measured by using a microtiter bacterial agglutination test. They were found in all sera examined, including normal human sera. It was comfirmed that a microtiter bacterial agglutination test on P. acnes is found to be an easy and satisfactory method for the measurement of antibody to P. acnes. The agglutinin titers of tumor patients, particularly hepatoma and gastric cancer patients, were significantly lower as compared with those of normal human sera. Antibody titers in leprosy patients were somewhat lower when compared with those in normal human sera. Antibody titers of lepromatous type of leprosy patient were lower than those of tuberculoid type. However, antibody levels were the same in schizophrenia patient and normal human. No correlation between antibody titers and age or sex of the patients and normal human was found.