The formation of new hair follicles was quantitatively demonstrated in the tail skin of adult mice in the course of a 2-stage carcinogenesis experiment with 7,12-dimethylbenz(a)anthracene as an initiator and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate as a promoter, as well as experiments with 12-O-tetradecanoylphorbol-13-acetate alone. Two kinds of follicular neogenesis could be distinguished. The most frequently encountered type was characterized by the organization of new follicles from the upper neck and orifice regions of already existing follicles. During their development, these new follicles remained in close apposition to the original follicles but, after having reached a critical size, split off to form fully independent follicles. In the 2nd type of follicular neogenesis, which occurred very rarely, the new follicles seemed to arise directly from the epidermis between 2 sets of hair triads. These follicles never reached their final stage and did not produce hairs. The formation of new hair follicles may be explained by a dedifferentiation of epidermal cells caused by the tumor promoter. Because of the paucity and advanced stage of the papillomas formed in tail skin after long-term treatment with 12-O-tetradecanoylphorbol-13-acetate, no reliable comment as to whether the papillomas derive from the hair follicle can be made.