A method allowing isolation and microbiological analysis of individual pilosebaceous units (follicles) was used to study biopsies of back skin obtained from volunteer acne vulgaris patients. The main microbial groups isolated were members of the genera Propionibacterium, Staphylococcus and Pityrosporum. The incidence (and mean density) of these organisms in 140 normal follicles was 12% (2.6 X 10(5) per follicle), 4% (5.5 X 10(3) per follicle) and 13% (10(2) per follicle) respectively. Colonized follicles were not distributed evenly amongst the subjects studied. The results are analysed and discussed from an ecological standpoint.