The minimum inhibitory concentration (MIC) of 12 antibiotics was determined for four strains of staphylococci (representing the three major species) under aerobic and anaerobic conditions by using the broth dilution method. Nine of the antibiotics showed no significant difference between aerobic and anaerobic MIC for any of the four cultures. Gentamicin and kanamycin showed a small, but significant, increase in MIC for at least two strains under anaerobic conditions. Trimethoprim was less effective against all strains under anaerobic conditions.