A DNA probe was developed by inserting, in the SmaI site of pBluescript sK, a 528-bp fragment of the gene responsible for intrinsic resistance to beta-lactam antibiotics in Staphylococcus aureus (mec determinant). The mec probe provided a useful tool for the rapid identification of the intrinsic resistance trait and for establishing guidelines for testing the in vitro susceptibility of S. aureus to beta-lactams.