Epidermal proliferation was studied in the guinea-pig ear in vivo using the incorporation of [3H]thymidine into DNA as a measure of cell proliferation. Twenty-four-hour pretreatment of the skin topically with nanomolar amounts of either leukotriene B4 (LTB4), 12-hydroxyeicosatetraenoic acid, ionophore A23187, or the epidermal mitogen 12-O-tetradecanoylphorbol-13-acetate induced dose-dependent increases in epidermal proliferation. The stimulatory effect of LTB4 was stereospecific since a number of biologically inactive LTB4 stereoisomers had no effect.