Mg2+-availability in Staphylococcus aureus cells decreased significantly with increasing NaCl concentration in growth media. Cells grew in a NaCl-free, Chelex resin-treated complex medium only if the medium was supplemented with 50 microM MgCl2, while, growth was limited when the medium was further supplemented with 1.0 M NaCl. Cells grown in such a high-NaCl/low-Mg2+ medium exhibited the morphologic abnormality of larger than normal cells. Both sufficient growth and normal cell morphology were restored by increasing Mg2+ concentration in a high-NaCl medium, or by supplementation with either CaCl2 or MnSO4 in a high-NaCl/low-Mg2+ medium. Supplementing with BaCl2, SrCl2 or FeSO4, however, had no effect. These results indicate that Ca2+ and Mn2+ might play some essential role in the growth of Staphylococcus aureus in a high-NaCl/low-Mg2+ environment.