Endemic hospital strains of Staphylococcus aureus are often resistant to mercury (Hg) ions (Moore, 1960). Determinants for resistance to Hg and other heavy-metal ions, such as cadmium (Cd) and arsenate (As), are often on the penicillase plasmid (Richmond and John, 1964; Dyke, Parker and Richmond, 1970) and strains resistant to all three metals often have multiple resistance to antibiotics (Dyke et al., 1970; Nakahara et al., 1977) and produce large amounts of penicillinase (Richmond et al., 1964). Different penicillinase plasmids harbour different heavy-metal resistances (Novick and Richmond, 1965; Dyke et al., 1970; Lacey and Rosdahl, 1974), and changes in metal-resistance pattern might be expected to reflect changes in the occurrence of plasmids in S. aureus. So far, investigations of metal resistance have been performed only on strains isolated during a short period or from a few hospitals (Dyke et al., 1970; Nakahara et al., 1977). The investigations described here were performed on bacteriaemia strains that were epidemiologically unrelated (Jessen et al., 1969) and had been isolated during a long period, 1957-74 (Rosendal el al., 1976b).