A study was made of the effect of antibiotics on staphylococci from patients in a mental hospital where drugs had been used infrequently and the staphylococci were largely susceptible. Erythromycin, to which all cultures before treatment were susceptible, caused a complete suppression of the carrier state, and only a few resistant strains appeared subsequently. Penicillin, to which 10 per cent of strains were resistant before treatment, caused a drop in the carrier rate to 60 per cent, but the fall was limited at this point by an increase in strains resistant to penicillin. Tetracycline, to which 4 per cent of strains were resistant before treatment, caused a fall in carrier rates to 20 per cent at which point increase in resistant strains prevented further reduction. Following treatment, the carrier rates in treated patients increased and the frequency of resistant strains diminished. Treated non-carriers and non-treated carriers on the same ward did not change during therapy experiments. Successive treatment of patients with penicillin and tetracycline caused the appearance of a group of cultures resistant both to penicillin and tetracycline, of which a majority did not react with phage (no type). A high proportion of strains of phage type 52–80 susceptible to penicillin and tetracycline initially present in the ward completely disappeared after use of these agents.