Three Staphylococcus aureus strains (303, 18Z and TG), exhibiting various patterns of survival within abscesses, were significantly more sensitive to the bactericidal activity of oleic acid during the log phase of growth than at other stages of the growth cycle. Cells entering the stationary phase showed diminished sensitivity to the fatty acid. These changes were reflected by changes in the LD50 and also by differences in the rate of killing by oleic acid. Additional changes were noted: the rate of killing by oleic acid declined over a 4-day period; a progressively greater proportion of the staphylococcal population became resistant to even high concentrations of oleic acid; from the fourth day onwards c. 50-55% of the cocci were totally resistant to the fatty acid. Strains 303 and 18Z became more sensitive to mono-olein during the log phase of growth, but strain TG was very resistant to mono-olein throughout the growth cycle. Growth in the presence of glycine 6% to reduce cross-links in the peptidoglycan did not alter bacterial sensitivity to oleic acid. However, all three S. aureus strains exhibited significant increases in membrane fluidity during the log phase of growth, but upon entering the stationary phase membrane fluidity again decreased. Concomitant changes in carotenoid content occurred during the growth cycle, but these changes did not appear to be solely responsible for the changes in sensitivity to the lipids.