Cells of Bacillus subtilis mutant 1508C were lysed by sodium ion concentrations above O. 1%. The wild strain was not affected by these conditions. Monovalent cations such as K^+ , Li^+, and Cs^+,also were effective in lysis of the mutant cells. On the other hand, divalent cations such as Ca^<2+> , Mg^<2+>, and Mn^<2+> protected the cells from the lytic action of Na^+. Na-induced lysis was inhibited by the addition of antibiotics such as kanamycin or chloramphenicol, while antibiotic; D-cycloserine (an inhibitor of bacterial cell wall synthesis) did not inhibit the lytic process. Autolysis by NaCl occurred even in cells which were osmotically protected with 0.8 M sucrose, while the protoplasts derived from the mutant strain were not lysed by the addition of NaCl under same osmotic protection. These results suggest that the rapid lysis of mutant 1508C induced by NaCl is due to the action of autolysin(s).