Carriage of resistant propionibacteria is one cause of poor therapeutic outcome in some antibiotic treated acne patients. For five years, we have monitored the carriage of resistant strains by directly inoculating samples obtained with moistened swabs from facial skin onto media containing selective antibiotics. To date, 2,467 patients have been screened. In 1992, 38.1% carried resistant strains. Erythromycin resistance (28.8% of patients) was the most common. The overall prevalence of resistant strains did not change significantly between 1992 and 1994. However, by 1995, 48.5% of patients carried resistant strains and by November 1996 the number had further increased to 61.8% (P<0.001, χ 2 ). The prevalence of tetracycline resistant strains has risen steadily from 17.8% in 1992 to 26.0% in 1996. In contrast, the prevalence of erythromycin resistant strains increased by 8.8% between 1992 and 1995 but by 15.8% between 1995 and 1996. Most of these isolates are cross resistant to clindamycin (88.9% in 1996). MIC determinations on 100 resistant and 25 fully sensitive strains collected during 1996 show changes compared with strains collected up to 1991. Paradoxically, the modal MIC for strains resistant to tetracycline and doxycycline has dropped fourfold whereas the modal minocycline MIC has doubled. A significant number of isolates with intermediate MICs have emerged. These changes may reflect the greater use of minocycline compared to other tetracyclines in the last decade.