Seven strains representative of the five serotypes of group B streptococci were converted to the L-phase by means of penicillin in a liquid medium. The conversion and adaptation to growth in the L-phase of the group B, type III strain 76-043 were studied in detail. This L-phase has been subcultured serially 204 times in liquid medium containing penicillin. Stability of the L-phase was evident by 195 serial subcultures in the absence of penicillin without reversion to the parent bacterial phase. Studies of the kinetics of growth in the cell-wall-defective state showed a lack of correlation between the number of viable units and the turbidity of the culture, as well as a rapid decline in the number of colony-forming units once the minimum pH of the culture was reached. Addition of Na2HPO4 or K2HPO4 to the growth medium did not prevent the sharp decline in the number of viable units. Group B and type III antigens continued to be detected in culture supernatants but not in HCl extracts of cells of the L-phase of strain 76-043. These immunological markers confirmed the precise bacterial origin of the L-phase.