In this study, the degree of tolerance was determined in several populations of Staphylococcus aureus isolates. The degree of tolerance of a staphylococcal strain can be established in a reproducible way by exposing the strain to increasing concentrations of a ,-lactam antibiotic and determining the number of surviving bacteria at each concentration. The The efficacy of antimicrobial treatment in infections caused by staphylococci with a high MBC:MIC ratio was studied several times. A negative correlation between tolerance and antimicrobial response was demonstrated in some (3,8,10, 11) but not all (6, 7) cases. The diverse results obtained in these studies may be based on differences in laboratory conditions used to demonstrate the phenomenon. In a previous study, we have demonstrated that the percentage of surviving bacteria exposed to high concentrations of a P-lactam antibiotic is reproducible constantly within certain limits (5). In the present study, we attempt to indicate the threshold value between susceptible and tolerant strains by determining the tolerance percentage of a number of S. aureus strains. To investigate whether the prevalence of tolerance has increased in the last few decades, two collections of older strains were studied as well.