SUMMARY: Thirty-eight strains of Corynebacterium acnes were used in a systematic study of biochemical and physiological reactions in an effort to define and clarify some of the characters of this species. Strains were isolated both from lesions in acne vulgaris and from normal skin. The characters which were positive for all 38 strains were: gelatin liquefaction within 20 days, catalase production, ammonia production in peptone broth, tributyrm hydrolyis, α-haemolysis (green) in blood agar containing human or rabbit red blood cells, acid production from fructose, glucose, galactose, mannose, glycerol and trehalose.