The serological identification of Propionibacterium acnes, P. granulosum, and P. avidum, using trichloroacetic acid extracts, is described. With antisera prepared against reference strains, the method has been tested on 142 strains recently isolated from human skin. All except two of the strains could be identified serologically, and there was excellent agreement between the serological results and the fermentation pattern of the strains. Two serological types of P. acnes and two of P. avidum were identified, but only one of P. granulosum.