We used population analysis to examine the effects of Triton X-100 on the level of resistance to oxacillin of 18 methicillin-resistant Staphylococcus aureus. In the presence of 0.02% Triton-X-100, 17 formerly methicillin-resistant strains exhibited enhanced sensitivity to oxacillin. One homogenous isolate, KSAF1 was barely affected by the Triton X-100. Sensitivities of lysostaphin, 51 kDa N-acetylglucosaminidase and 62 kDa N-acetylmuramoyl-L- alanine amidase to heat-inactivated cells were not affected when the bacteria were grown in 0.02% Triton X-100. Our data, together with those of a previous study, suggested that Triton X-100 alters the resistance level of methicillin-resistant S. aureus influencing a factor (s) other than PBPs, bacteriolytic enzymes, or femAB products.