The synthesis of beta-lactamase in response to 2-(2'-carboxyphenyl)-benzoyl-6-aminopenicillanic acid as inducer was studied in Staphylococcus aureus. The inducer was not detectably hydrolyzed by beta-lactamase and had minimal antibacterial activity. The kinetics of induction showed a lag of 4 to 6 min in a nutrient broth medium and 8 to 12 min in a defined medium, followed by constant differential rates of synthesis of beta-lactamase. The differential rate of beta-lactamase synthesis in nutrient broth was unaltered by supplementing the medium with glucose, galactose, lactose, arabinose, glycerol, or sucrose. Variations in the partial pressure of oxygen did not alter the differential rate of synthesis of beta-lactamase over the range 18 to 50% oxygen in nitrogen. Even when the rate of growth was considerably reduced by high-oxygen tension, the differential rate of synthesis of the enzyme remained the same. The differential rate of beta-lactamase synthesis at low inducer concentration increased after a shift down in growth rate. The effect was observed with several inducers and under different nutritional conditions, but was always preceded by a change in growth rate. It is suggested that the change in growth rate itself causes the increase in differential rate of beta-lactamase synthesis.