The energy-dependent efflux of erythromycin (Er) in staphylococci is due to the presence of msr A, which encodes an ATP-binding protein. MsrA is related to the multi-component ATP-binding cassette (ABC) transporters which characteristically also contain membrane-spanning domains. Since MsrA functions in a heterologous host in the absence of other plasmid-encoded products, the requirement for a transmembrane (TM) complex might be fulfilled by hijacking a chromosomally encoded protein. Two genes, stpA and smpA, were identified upstream from msrA on the original Staphylococcus epidermidis plasmid, encoding an ATP-binding protein and a hydrophobic TM protein, respectively. Sequences highly similar to stpA and smpA (stpB and smpB) were also found adjacent to a chromosomal copy of msrA in S. hominis. In Southern blots, internal fragments of stpA or smpA hybridized to the chromosome of the Ers S. aureus RN4220. Cloning and sequence analysis of the region identified revealed the presence of two genes, stpC and smpC, related to stpA and smpA. The deduced amino-acid sequences of the gene products showed that StpA and StpC were 85% identical, whereas SmpA and SmpC were 65% identical. A gene similar to msrA was not present in the S. aureus chromosome. There was no further sequence similarity outside these conserved regions. These results indicate that the chromosomes of S. hominis and S. aureus contain sequences encoding a potential TM protein with which MsrA might interact.