Experiments with Staphylococcus aureus and Streptococcus pyogenes have shown that when cells are exposed to a bactericidal concentration of benzylpenicillin or am-picillin and the antibiotic then inactivated with β-lactamase the count of the surviving viable cells (colony-forming units) remains essentially constant for a period of approximately 1.5–3 h before increasing at a normal rate. No such recovery period was observed, however, when Escherichia coli or Proteus mirabilis was exposed to a bactericidal concentration of ampicillin and the antibiotic then inactivated. Microscopic observation of individual surviving cells of S. aureus following exposure to benzylpenicillin showed that in some cases resumption of cell division was delayed as long as 4 h. However, other cells in the same population showed no recovery period and resumed division within 30 min of inactivation of the antibiotic. In the latter case, the recovery period which is observed in terms of viable count may represent the period of time required for individual surviving cells to divide and give rise to normal aggregates before these fragment to produce new colony-forming uni