Suppression of epidermal proliferation by ultraviolet light, coal tar and anthralin.

360 nm ultraviolet light (UVA) plus coal tar depresses mitosis and DNA synthesis in epidermal cells. UVA has no effect by itself. In some models coal tar alone can partially depress DNA synthesis. 254 nm ultraviolet light (UVC) and 290--320 nm ultraviolet light (UVB) do not enhance depression of DNA synthesis by coal tar. Dithranol will suppress mitosis and DNA synthesis of epidermal cells but there appears to be no photo-activation by UVA. Dithranol suppression and suppression of DNA synthesis by UVB or UVC are additive when used together.