A low affinity penicillin binding protein (PBP2') present in methicillin resistant strains but absent in isogenic sensitive strains of Staphylococcus aureus is responsible for the phenotypic expression of the methicillin resistance. An additional factor controlling methicillin resistance is known to map on the chromosome at the insertion site omega 2003 (chr::Tn551). The influence of this factor on PBP2' synthesis was analysed. Isogenic methicillin sensitive and resistant mutants derived from a Staphylococcus aureus strain carrying methicillin resistance factors and the omega 2003 (chr::Tn551) insertion were characterized by population analysis and restriction analysis of the DNA surrounding the Tn551 insertion site, and their PBP2' content was determined. Both methicillin resistant and sensitive mutants produced levels of PBP2' similar to those of the original methicillin resistant parent. The mere presence of this PBP2' could therefore not be solely responsible for the phenotypic expression of methicillin resistance. The nature and site of action of the additional factor determined by the omega 2003 (chr::Tn551) site controlling expression of methicillin resistance still remains unknown.