Recent observations have shown that mammals possess a limited capacity for regeneration of inner ear sensory epithelia. It is clear, however, that a mitogenic growth factor will be necessary to up-regulate this capacity before clinical application becomes feasible. This study used in vitro cultures of adult mouse vestibular organs for assessing the mitogenic effect of transforming growth factor alpha (TGF-alpha). Sixty-one utricles and cristae were maintained in culture for 7 to 8 days. Neomycin was used to damage the hair cells. Autoradiography permitted identification of any cell which had undergone mitosis during the culture period. The proliferative response was compared in organs exposed to TGF-alpha and those maintained in the basic culture medium only. The results demonstrated that TGF-alpha significantly increased cell proliferation in the sensory epithelia and also in the marginal zones surrounding them. This finding provides a scientific basis for the concept that inner ear hair cell damage in humans may someday be reversible pharmacologically.