A study of the pathogenicity of the Enterococcus and Methicillin resistant S. aureus (MRSA) was made of 781 cases with gastroenterological surgery. The results obtained were summarized as follows. In this study 215 strains were isolated from clinical specimens. In these strains 13% was isolated from the postoperative mixed infection and only 7% occurred in single infection. Most of the single infections by the Enterococcus occurred in the cases with changes in host resistance, such as terminal cancer, disorder of the glucose tolerance, malnutrition and/or organ failures. Seventy-three strains were isolated from clinical specimens. In these strains 10% were the postoperative mixed infection and 40% were single infection. Incidence of the infection by the MRSA was significantly high compared with that by the Enterococcus. Single infection by the MRSA occurred even in the cases with normal host resistance. Strains producing enterotoxin C (single or plus A) were frequently isolated in MRSA enteritis or pneumonia. However, 60 percent of the patients in whom MRSA were detected in their abdominal drains had no signs of infection. These results suggest that the occurrence and severity of the MRSA infection relate with site of the infection and production of the exotoxin.