Lymphokines may alter epidermal growth and differentiation contributing to changes such as acanthosis and hyperkeratosis. The main in vivo effects of lymphokines on epidermal mitotic activity were therefore investigated. Guinea pigs were injected intradermally with antigen-stimulated lymphocyte culture supernatants and a partially purified lymphokine preparation in phosphate buffered saline (PBS) 18, 24, 36 and 48 hr prior to biopsy. Control sites were injected with unstimulated supernatants and PBS respectively and the mitotic activity determined by use of a stathmokinetic agent. Both lymphokine injected areas and controls showed significantly increased mitotic indices compared to untreated skin which was apparent only at 24 hr. However mitotic activity in lymphokine lesions was significantly higher than in control lesions. There was no difference in the effect on mitotic activity between PBS and unstimulated culture supernatants. Lymphokine lesions at 24 hr also exhibited marked epidermal edema and acanthosis compared to minimal changes in controls. A variable patchy parakeratosis developed between 18 and 24 hr in areas injected with partially purified lymphokine but not in control sites or after injection with unpurified supernatants. The lymphokine-induced inflammatory infiltrate was mild and consisted mainly of neutrophils not differing significantly from that of the control lesions. This strongly suggests that lymphokines induce an alteration in epidermal kinetics and keratinization by a direct effect on keratinocytes and not indirectly via the dermal inflammatory infiltrate.