The prevalance of antibiotic-resistant strains of Staphylococcus aureus was studied in three Birmingham hospitals. In a general hospital periodic surveys showed a progressive decline in the proportions of patients with Staph. aureus in their noses which were resistant to tetracycline, erythromycin, and kanamycin. This change was associated with a progressive reduction in the use of tetracycline without an overall reduction in the use of antibiotics. There was no similar decline in resistance of staphylococci isolated in a hospital for skin diseases. In a burns unit there was a sudden large reduction during September, 1978, in the proportions of Staph, aureus from burns which were resistant to tetracycline, methicillin, cephaloridine, erythromycin, lincomycin, novobiocin, gentamicin, and kanamycin, and in the proportions of multiresistant strains (resistant to penicillin, tetracycline, erythromycin, kanamycin, methicillin, novobiocin, cephaloridine, and lincomycin). This change was associated with a reduction in the number of patients and in the use of antibiotics; tetracycline was not in use except during one month of the study. Strains of Staph. aureus resistant to these antibiotics became common again in the burns unit when a larger number of patients were admitted and more antibiotics were used in the wards.