The effects of the immunosuppressive drugs azathioprine, prednisolone and cyclosporin A, and UV irradiation on scheduled DNA synthesis, representing mitotic activity, and unscheduled DNA synthesis, representing DNA repair activity, in the skin of hairless mice were determined autoradiographically following intradermal administration of [3H]-thymidine. Azathioprine and prednisolone, and, to a lesser extent, cyclosporin A, were found to impair normal repair of UVR-induced DNA lesions in epidermal keratinocytes. Prednisolone, but not the other two drugs, further suppressed the normal delayed mitotic response in the skin following UVR. The depressive effects of these drugs, particularly the combined azathioprine/prednisolone regime, on scheduled and unscheduled DNA synthesis are considered likely to predispose skin to mutagenic and carcinogenic processes.