The effect of different categories of membrane stabilizers on K+ loss and growth has been characterized in a culture of Staphylococcus aureus. Chlorpromazine, thiopental and tetracaine at low concentrations produced a marked inhibition of K+ loss and an equivalent increase in the K+ contents of S. aureus. Whereas the inhibitory effect of chlorpromazine on K+ loss was observed at lower than bacteriostatic concentrations of the drug, thiopental had no effect on growth in the concentration range where K+ loss was maximally inhibited. It is concluded that the bacteriostatic action of chlorpromazine is probably not related to its membrane stabilizing effect only.