Damage to the epidermis occurs with a single painting of 0.6% methylcholanthrene-in-benzene and a much more severe damage occurs with the addition of 1000r irradiation. By previous plucking, known stages of hair proliferation in prescribed areas are obtainable at the time of treatment. C57 Black mice are used in the expt. Maximal cytological and histological damage occurs in resting (telogen) areas; much less damage occurs in active (anagen IV) areas. Furthermore, restoration of sebaceous glands and repair of epidermal damage are associated with active hair proliferation. Cells of the external sheath may play a role in the resistance to epidermal damage and in the capacity for repair which are associated with active hair stages as opposed to resting hair stages.