Abstract
The rate of death of certain "zone-sensitive" bacterial strains exposed to optimal concns. of penicillin may be markedly reduced by the following addition of high concns. of the drug. On the other hand, if these organisms are first exposed to high concns., and the suspension is then diluted back to the optimal level, the bacteria continue to die at the slow rate imposed by their original exposure. Penicillin at different concn. levels thus exerts 2 distinct effects on zone-sensitive bacteria. Working hypotheses as to the mode of action of penicillin are discussed.