Against 29 strains of methicillin-resistant Staphylococcus aureus (MRS A, MIC ≥12.5μg/ml) clinically isolated in three hospitals in Osaka and 6 standard strains (MSSA), the antibacterial activity of 10 antibotics, methicillin, ampicillin, cefotiam, cefuzonam, gentamicin, minocycline (MINC), doxycycline (DOXY), fosfomycin, vancomycin (VCM) and imipenem/cilastatin sodium and the bactericidal activity of 4 disinfectants, benzalkonium chloride (BAC), chlorhexidine digluconate (CHG), alkyldiaminoethylglycine (TG) and povidone iodine (PVP-I) were investigated by time-kill kinetic studies to clarify susceptibility to the drug.<BR>MICs of 10 antibiotics against 6 MSSA strains were between 0.006-1.56μg/ml except in the case of FOM. BAC and PVP-I had strong bactericidal activity against MSSA strains, but CHG and TG did not kill MSSA strains within 10 min.<BR>The MICs₅₀ of MINO, DOXY and VCM were 0.39, 0.78 and 0.78μg/ml, but those of the other antibiotics tested were above 25μg/ml. The MICs₈₀ of DOXY and MINO were 3 times larger than those of MICs₂₀ in a ratio of two step dilution, but The MIC₈₀ of VCM was equal to its MIC₂₀. These results indicated that DOXY, MINO and VCM showed stronger antibacterial activity than the other antibiotics tested in this study.<BR>BAC and PVP-I killed standard strains effectively. PVP-I, mainly used in wound sites, had the strongest bactericidal activity against MRSA strains. The bactericidal activity of BAC against MRSA strains was stronger than that of CHG and TG. By addition of 10-20% ethanol or of 0.01% sodium carbonate to BAC, its bactericidal activity against the MRSA strains was notably increased as compared with 0.1% BAC alone.