Cultures of tetracycline-treated Staphylococcus aureus exhibited monophasic steady-state growth curves similar to that observed for tetracycline-treated Escherichia coli. Apparent growth rate constants of the respective drug-treated cultures showed the same formal dependence on drug concentration, which was linear at a low concentration but asymptotically approached zero at higher concentration levels and implied the saturation of a limited number of receptor sites engaged in microbial protein synthesis. The relative potency of tetracycline action of S. aureus/E. coli was 6.50:1 at 37.5°C and pH 7.05. This is attributed to relative differences in drug permeation and/or binding affinity for biophase receptors in the respective organisms. It is concluded from kinetic dependencies of growth inhibition of the cultures that tetracycline has the same mode of action on S. aureus and E. coli. It is bacteriostatic at concentrations below the minimal inhibitory concentration level but bactericidal at the higher concentration levels.