The distribution of sensitivity to 9,3"-diacetylmidecamycin (MOM) was examined of 10 strains of Staphylococcus aureus isolated from foci in our hospital. A level of less than 3.13 mcg/ml was seen distributed in the majority of 7 cases, and it could be assumed that MOM would be sufficiently effective against infections by Staphylococci in clinical practice. In the comparison with other antibiotics popularly used, the sensitivity of the 10 strains to them was comparable to those against MDM, but were slightly inferior to cephalexin (CEX) and lincomycin (LCM), and slightly better than ampicillin (ABPC). The evaluation of clinical results revealed that, of the 20 cases with surgical and superficial infections, 6 and 10 were markedly and moderately effective, respectively, while 3 and 1 were fairly and poorly effective, respectively. It was therefore concluded to be an effective remedy for treatment of surgical and superficial infections in which Gram-positive cocci share the majority of the etiological factors. Of the side effects, there was none showing symptoms in digestive tracts or abnormalities such as rash, nor was there any patient for whom further administration of the agent had to be suspended due to side effects. No abnormality was seen in the test results between the premedication and postmedication findings on the hemogram, or hepatic and renal functions. It was therefore conjectured that this agent has a large margin of safety in its application.