AbstractThis paper demonstrates that epidermal cells in culture produce an activity which can increase the frequency of Ia+epidermal Langerhans cells (LC). This was achieved by treating mice topically with a mixture containing supernatant derived from primary culture of murine epidermis (ES) and a synthetic corticosteroid, triamcinolone acetonide (TAG). The presence of the supernatant in the mixture partially protected the Ia+LC from depletion by the steroid. The Ia+LC frequency increasing activity was measured as the difference between the Ia+LC frequency due to treatment with steroid mixed with supernatant and the Ia+LC frequency due to treatment with steroid mixed with negative control medium. The mean frequency of Ia+LC in epidermis treated with TAC mixed with ES was 606(SD 43) cells/mm2, as compared with 486 (SD 68) cells/mm2in the epidermis treated with TAC mixed with control medium. The activity appeared to be caused by (a) proteinaceous factor(s). A fraction of ES which was retained above a ≥10 KDa molecular weight cut‐off membrane was capable of partially protecting Ia+LC frequency from TAC depletion. Supernatants from cultured lymph nodes, dermis as well as the squamous cell carcinoma lines T7 and T79, but not the human osteosarcoma cell‐line 143B, also contained similar activities. We demonstrate that GM‐CSF also increased the number of Ia+epidermal LC when applied topically to mouse skin in this system. Therefore, using this Ia+LC frequency modulation system, we propose that GM‐CSF is one example of a cytokine which may be involved in the regulation of Ia+LC numbers in epidermis and that epidermal cells produce factors which can increase the number