Susceptibility of 983 isolates of Staphyloccus aureus to tetracycline, doxycycline and minocycline was determined in vitro. Minocycline was shown to be more active than doxycycline, which in turn was shown to be slightly more active than tetracycline. 77% of the isolates which were resistant to tetracycline were also resistant to doxycycline, whereas only 4% of the tetracycline-resistant isolates were resistant to minocycline. The in vivo use of tetracycline correlated with increased in vitro resistance of S. aureus to tetracycline and doxycycline. A correlation between use of tetracycline and in vitro resistance to minocycline was not demonstrated.