During the 3-years period from 1989 to 1991, 15 methicillin-resistant Staphylococcus aureus (MRSA) strains were isolated from 13 mares with metritis in Hidaka District, Hokkaido, Japan. All these isolates proved to possess mec A gene and showed MICs of 25-100 μg/ml for methicillin. The isolates were found to be multidrug resistant to erythromycin, tetracycline and aminoglycoside antibiotics as well, to exhibit the same antimicrobial sensitivity patterns and to have common characteristics of being phage type III, coagulase type IV and enterotoxin type A. The organisms therefore were strongly suggested to have originated from the same source. Then, bacteriological examinations were performed at a stud farm suspected to be the source of MRSA infection. As the result, MRSA was not detected from reproductive organs of stallions, their environment or farm employees but was done consistently from cutaneous lesion of pastern in the hind leg of a stallion in that farm over a period longer than two months. The MRSA isolates obtained from the skin lesion of the stallion were found to be identical in the characteristics with the isolates from metritis in the mares. It was thus inferred that the organism was persistently infecting the cutaneous lesion in the stallion eventually contaminated the stud farm environment, resulting in infections in the mare received for mating.