Mupirocin is a topical antimicrobial agent that is used for the treatment of skin and postoperative wound infections, and the prevention of nasal carriage of methicillin-resistant Staphylococcus aureus (MRSA). However, the prevalence of mupirocin resistance in S. aureus, particularly in MRSA, has increased. High-level mupirocin resistance, with MICs >or=512 microg ml(-1), is mediated by the ileS-2 gene, which is located on conjugative plasmids. In the present study, we investigated whether mupirocin influences the expression of the ileS-2 gene responsible for high-level mupirocin resistance, and we present some evidence that this gene is not upregulated but constitutively expressed in S. aureus.