Addition of novobiocin and its benzoic acid moiety, 3-(3-methsl-2-butenyl)-4-hydroxybenzoic acid to growing cells of Staphylococcus aureus, most of which contain a penicillinase plasmid bearing determinants for penicillinase production, results in a decrease in penicillinase-producing cells and an increase in cells that do not make the enzyme. Two other benzoic acid derivatives, methyl 3-(3-chloro-2-butenyl)-4-hydroxybenzoate and ethyl 4-hydroxy-3-isopentylbenzoate cause a similar population shift. The induced change in ratio of penicillinase-producing to nonproducing cells is due to a selective action by the compounds in favor of cells which do not make penicillinase. There is no evidence that the compounds eliminate plasmids from the cells.