PURPOSE: Low-affinity penicillin-binding protein PBP 2a encoded by mecA is closely related to methicillin resistance in staphylococci, and the expression of PBP 2a is controlled by regulator elements encoded by mecR1 and mecI. Deletion or mutation which occurred in mecI is considered to be associated with constitutive production of PBP 2a. We investigated the distribution of mec regulator genes and the presence of the mutations in mecI among mecA gene-positive methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase negative Staphylococcus (MRCNS) strains. METHODS: A total of 28 MRSA and 26 MRCNS clinical strains were isolated at Chung-Ang University Hospital. The distribution of mec regulator genes and the presence of mutations in mecI were analyzed using polymerase chain reaction and direct sequencing. RESULTS: In 28 MRSA and 26 MRCNS, only mecR1A-positive pattern (type lll) was detected in 53.6% of MRSA and 46.4% of MRCNS. The mecR1 (mecR1A and mecR1B) and mecI-positive pattern (type l) were detected in 42.3% of MRSA and 38.5% of MRCNS. In 19.2% of MRCNS was type lV in which no mec regulator genes were detected. Our results showed that a greater genomic variation existed in MRCNS than a MRSA. Results in direct sequencing of mecI revealed that mecI gene tested in our study did not harbour mutations and deletions. There was no correlation between the level of resistance and the presence or absence of mec regulator genes. CONCLUSION: The induction of methicillin resistance and the variability of phenotypic expression of methicillin resistance suggested that additional factors on the chromosome are involved.