The effects of osmotic stabilizers on swelling and growth of L-form NC7 derived from Escherichia coli K 12 were examined. The presence of impermeable solute , such as NaCl or KCl, in the suspended medium were effective as osmotic stabilizer, while permeable solute, such as glycerol were ineffective . On the other hand, when sucrose was used as osmotic stabilizer, in cells upshocked with 0.2 M sucrose, rapid decrease in optical density and then slow restoration were observed. In the upshocked cells, potassium ion had protective effect against changes of turbidity. From colony forming units at 1 hr after upshock, it was shown that there was no srgnificant different between cells upshocked with 0.2 M sucrose alone and with 0.2 M sucrose plus 0.34 M KCl. Low osmolarity (0.17 M) of cation, such as NaCl or KCl, was not sufficient to protect fragile L-form cells unless 0.2M sucrose was added in the incubation medium. In combination with NaCl and KCl at high concentration (0.4M), there are competition between Na and K ions on growth, suggesting that in the combination with calcium (1 mM), Na ion also play a important role on growth of L-form NC7 as well as K ion.