Summary Mice, inoculated intraperitoneally with lethal doses of group A streptococci, yielded L forms as well as streptococci from peritoneal exudate and heart blood at death. These L forms propagated readily and were recovered on penicillin-free medium after separation from the streptococci by differential filtration. They were shown to be derived from the original streptococcus, and usually appeared in mice inoculated with streptococci of relatively low virulence.