Harold, F. M. (National Jewish Hospital, Denver, Colo.). Stabilization of Streptococcus faecalis protoplasts by spermine. J. Bacteriol. 88:1416-1420. 1964.-Lysis of protoplasts of Streptococcus faecalis subjected to osmotic shock was prevented by the presence of 10(-3)m spermine and other divalent cations. Protein and nucleic acids were largely retained, but compounds of low molecular weight were discharged into the medium and the capacity for glycolysis was lost. Under these conditions, spermine was bound to the protoplasts. It could not be removed by washing with water or nonelectrolytes, but was displaced by salts, polyanions, and polycations. Removal of the spermine restored the osmotic fragility of the protoplasts, which could once again be protected from lysis by impermeant solutes. Protoplasts were also stabilized, in the absence of osmotic shock, by prolonged incubation with cations in 0.5 m sucrose. By either procedure, the protoplasts became resistant not only to osmotic lysis but also to sonic oscillation. It is concluded that the stabilization of protoplasts resulted from ionic binding of the cation to acidic sites on the external surface of the plasma membrane. This conferred upon the membrane additional mechanical strength, perhaps by the cross-linking of subunits, but did not alter its permeability to extracellular solutes.