The effects of immunosuppressive drugs on epidermal cell mitotic activity and the proliferative response of epidermis following ultraviolet radiation (UVR) were tested. Hairless (Skh-hr 1) mice were treated with immunosuppressive drugs at equivalent clinical doses with or without concomitant UVR (290-400 nm). Epidermal parameters measured were mitotic index (Im), rate of entry into mitosis (Fm), flash labelling index (FLI), rate of entry into DNA synthesis (Fs) and DNA content (flow cytometric analysis). In non-irradiated skin, prednisolone therapy depressed both mitotic activity and DNA synthesis; azathioprine and cyclosporin A had no effect; cyclophosphamide therapy increased the Fm and FLI values. Following repeated doses of UVR, there were enhanced mitotic activity and DNA synthesis in epidermis. Prednisolone therapy moderately depressed both proliferative responses; cyclophosphamide enhanced mitotic activity; azathioprine and cyclosporin A had no effect on these responses. The significance of these findings in relation to potential for increased susceptibility of skin to UV-induced carcinogenesis is discussed.