We evaluated a clinical strain of methicillin-resistant Staphylococcus aureus (MRSA), Mu50, for vancomycin susceptibility. Mu50 was isolated from a patient with infection of a surgical incision site which resisted vancomycin therapy. Mu50 showed a decrease in susceptibility to vancomycin, but this strain did not carry vanA or vanB or vanC genes as judged from PCR amplification. MICs of vancomycin against Mu50 and vancomycin-susceptible S. aureus FDA209P, S. aureus ATCC-29213, and MRSA H-1 were 8, 1, 1, and 1 microgram/ml, respectively by agar dilution and macro-broth dilution methods according to NCCLS. MIC values with agar dilution method using MHA + 20% horse serum, HIA, and BHIA agreed with the MIC values with micro- and macro-broth dilution method. Population analysis revealed that vancomycin concentration required for inhibition of ca. 10(7) cells of Mu50, S. aureus FDA209P, S. aureus ATCC29213, and MRSA H-1 were 36, 2, 2, and 2 micrograms/ml, respectively. These results showed that the activity of vancomycin against Mu50 was at least 8-fold decreased compared to that against S. aureus FDA209P, S. aureus ATCC29213, and MRSA H-1.