Propionibacterium species were quantified on the foreheads and cheeks of persons with and without acne in three age groups: 11 to 15, 16 to 20, and 21 to 25. Propionibacteria were virtually absent in the pubertal non-acne group compared to a geometric mean density of 114,800 per sq cm in the acne group. A similar sharp difference existed between the acne subjects and normals in the age range of 16 to 20 years: 85,800 organisms per sq cm compared to 588 per sq cm. Patients with acne and normal subjects over age 21 showed no difference in Propionibacterium levels. In acne patients, while there was a trend for lower levels, no significant difference was seen as the severity of inflammation increased.