The presence and sequences of genes that regulate the expression of methicillin resistance was investigated in 42 isolates of Staphylococcus aureus and 102 isolates of coagulase-negative staphylococci (CNS). PCR was used to detect mecA and the regulatory genes mecR1 and mecI. In a selected group of isolates, the sequences of mecI and the mec promoter region were also determined and compared with the sequences obtained from pre-MRSA strain N315. The genetic diversity of the collection was assessed by pulsed-field gel electrophoresis (PFGE). mecA was present in 21 S. aureus and 44 CNS. mecR1 was associated with mecA in all S. aureus and in all CNS, except two isolates of Staphylococcus haemolyticus. mecI was present in 48% of mecA-positive S. aureus and 50% of mecA-positive CNS. In six S. aureus isolates, mecI contained a termination codon at nucleotide 202 which would truncate the MecI protein. No mutation was found in the mecI gene of the four other S. aureus and 15 CNS sequenced. Seven isolates of Staphylococcus simulans had a single nucleotide substitution in the mec promoter region. Expression of methicillin resistance could be explained for all mecA-positive staphylococci with mutations within mecI or in the mec promoter region or in which mecI was deleted. However, the 'wild type' sequences observed in four S. aureus and eight CNS suggest that there is another mechanism for overcoming the repression of resistance caused by mecI.