In preventing the lysis of a slightly halophilic marine Vibrio alginolyticus, Na+ acted cooperatively with Mg2+, while K+ counteracted the effect of Na+ and Mg2+. When the cells were washed with a medium containing NaCl lower than that of the growth medium, the intracellular K+ was released from the cells and the minimum concentration of KCl for preventing cell lysis (lysis point for KCl) decreased in parallel to the concentrations of NaCl in the washing medium. The lysis points for KCl were increased by increasing NaCl in the growth medium. Rates of the initial rapid lysis measured by a stopped-flow method were dependent on the concentrations of KCl to which the cells were transferred. On the other hand, the lysis point for NaCl was unaffected by the concentrations of NaCl in the washing medium or in the growth medium. The results have been interpreted as indicating that Na+ prevented the cell lysis by providing the sufficient mechanical strength of the envelope against mechanical stress and that K+ prevented the cell lysis by balancing the internal osmotic pressure of the cells.