The minimal inhibitory concentration (MIC) of Propionibacterium acnes in seventy-five acne patients receiving long-term antibiotic therapy demonstrated the emergence of resistant strains. The mean MIC in thirty-three patients receiving long-term tetracycline was four to five times higher than that found in control groups of acne patients not receiving antibiotic therapy and controls free of acne. The average MIC for erythromycin was more than 100 times higher in those receiving long-term antibiotic therapy. In a second group of sixty-two patients, the clinical course and number of P. acnes were correlated with the presence of "resistant strains" defined as P. acnes with a tenfold increase in MIC to tetracycline or erythromycin. Patients with resistant strains had higher counts of P. acnes and clinically were not doing as well as those with sensitive strains.