In dermatophytosis, there is exocytosis of polymorphonuclear leukocytes (PMNs) toward the fungus-laden horny layer. To analyze this mechanism, we studied in vitro leukotactic properties of epidermal extracts prepared from lesions of experimental Trichophyton mentagrophytes infection in guinea pigs as well as fungus-derived chemotactic factors. Trichophyton mentagrophytes was found to release low-molecular-weight PMN chemotactic factors with its growth, like other microorganisms. Furthermore, it activated complement via the alternative pathway to produce complement-derived chemotactic factors in vitro. The epidermal extracts prepared from weakly inflammatory lesions of an early stage of T. mentagrophytes infection showed mild PMN chemotactic activity, which, as in those from irritated skin, was mostly due to the presence of low-molecular-weight chemotactic factors. After 8 days of infection, when prominent PMN migration took place together with the development of immune reactivity to fungus antigens, the epidermal extracts revealed strong leukotactic activity that showed a triphasic pattern by Sephadex G-75 chromatography similar to that observed in complement-activated serum. Since we could not demonstrate any deposition of immunoglobulins and complement on the fungal elements present in the horny layer, actual complement activation in vivo seems to occur after interaction of serum with soluble fungal components in the epidermis through both classic and alternative pathways. We think that the transepidermal migration of PMNs in dermatophytosis, together with contact sensitivity to fungal antigens, is responsible for induction of increased epidermopoiesis with resultant desquamation to eliminate the fungus-laden horny layer.