Methicillin resistance in S.aureus and S.epidermidis strains is primarily due to production of a new penicillin-binding protein PBP2' with extremely low binding affinity to most beta-lactam antibiotics. The structural gene for PBP2', mecA, is detectable in clinical specimens by using the polymerase chain reaction (PCR). Amplified target DNA of 630bp can be resolved on ethidium bromide-stained gels, and hybridized with a probe conjugated to alkaline phosphatase. Survey for the mecA gene in 304 staphylococci revealed a good correlation between the presence of mecA and cultivation on agar plates with 4 micrograms/ml of oxacillin, although 3% of sensitive S. aureus strains had the mecA gene. On the other hand, analysis of the regulatory genes (orf 1 and 2) of methicillin resistance was performed on methicillin-resistant S.aureus strains N315 and MR108, demonstrating that the genome of MR108 lacks orf 2 which encodes the repressor protein (Hiramatsu et al., see Ref.5). The regulatory genes of mecA were surveyed for 192 staphylococci by using PCR and allele-specific oligonucleotide probes: 76% of resistant S. aureus strains and 48% of resistant S. epidermidis strains possessed orf 1 corresponding to MR108 (constitutive-type strain), while the remainder of the resistant strains and two strains of sensitive S. epidermidis had two orfs of N315 (inducible-type strain). Furthermore, it appeared that mutation of the femA gene might not be an additional factor for expression of methicillin resistance. These observations suggest that mecA and its regulatory genes should be examined to understand how the genetic background contributed to the phenotypic expression of methicillin resistance in clinical strains.