Osmotic protection by the addition of sodium chloride to the level of 750 m.osmoles per kg was required to support an adequate growth of penicillininduced L-forms of on a serum agar medium. This requirement was not reduced by the addition of spermine or by the acidification of the medium.

L-form preparations that gave typical L-form colonies on serum agar containing 0·5-NaCl without added penicillin, but no colonies on isotonic serum agar, were tested for the ability to survive in the serum and tissues of mice. The L-forms were rapidly killed when incubated in mouse serum unprotected by the addition of NaCl, and disappeared rapidly after being injected into the bloodstream or peritoneal cavity. Osmotic lysis was probably the cause of their death and disappearance, since the mouse serum did not contain antibodies capable of killing the L-forms, and treatment with cortisone, which inhibits phagocytosis, did not affect the rate of their clearance from the peritoneum. Although the L-forms slowly underwent lysis in mouse urine, ones that were injected into the urinary bladder were probably also removed to a large extent by the voiding of the urine.

The findings show that L-forms were not virulent in mice.


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