The phenomenon of antibiotic-tolerance to 7 cell-wall-acting antibiotics and 5 aminoglycosides was studied in 35 strains of Staphylococcus aureus isolated from blood cultures. The minimal inhibitory concentration for antibiotic-tolerant S. aureus was in the susceptible range but the minimal bactericidal concentration was higher and often in the resistant range. Antibiotic-tolerance was common in strains of S. aureus and it involved not only antibiotics which inhibit cell wall synthesis but also the aminoglycosides. There was no uniform cross-tolerance among related antibiotics. Population analyses showed that the susceptibility of the cell population of antibiotic-tolerant S. aureus was heterogeneous and that the majority of cells were susceptible to the antibiotic tested. There was no difference in the magnitude or rate of killing of antibiotic-susceptible versus antibiotic-tolerant S. aureus.