Persistent suppression of bacterial growth by certain antibiotics was tested by periodic counts of viable organisms in a culture of Staphylococcus aureus that had been incubated in media containing drugs for limited periods of time and then removed by centrifugation. During short (2 hr) periods of exposure of test cultures to penicillin G, cephalothin, erythromycin, clindamycin, vancomycin, and tetracyline, effects on the growth of S. aureus were produced that persisted after removal of the drug for periods of 1.7-4.1 hr. A persistent antibiotic effect was not observed with gentamicin. The persistent effects of penicillin G and erythromycin were directly related to duration of exposure and concentration of drug, up to a point of maximal response. The maximal durations of bacterial suppression after exposure to penicillin G and erythromycin were approximately 2 and 5 hr, respectively. These effects were observed over a wide range of inocula.