Earlier investigations demonstrated that production and secretion of toxic shock syndrome toxin-1 (TSST-1) and total exoprotein by strains of Staphylococcus aureus were maximal when magnesium ion was limiting and diminished when the concentration of magnesium increased. This investigation studied the influence of magnesium concentration on production of total exoprotein and beta-lactamase by strains of S. aureus and Staphylococcus hemolyticus, isolated from the genital tracts of women. These strains were resistant to penicillins. Each organism was incubated in chemically defined medium with various concentrations of magnesium, and total exoprotein production and beta-lactamase activity in supernatants were determined. In all strains, total exoprotein production and beta-lactamase activity per bacterial cell were markedly increased in the presence of low concentrations of magnesium. When the concentration of magnesium was elevated, production of total exoprotein and beta-lactamase was decreased. Therefore, magnesium-deficient strains of S. aureus and S. hemolyticus may secrete more exoproteins and be more resistant to beta-lactam drugs than when magnesium is not limiting.