Ultraviolet A (UVA: 320-380 nm) radiation is an oxidizing carcinogen that has proved an ideal agent for demonstrating the oxidant inducibility of the mammalian heme oxygenase-1 (HO-1) gene. The UVA response in cultured human skin fibroblasts and other cell types is mediated by singlet oxygen and is strongly influenced by cellular reducing equivalents. Free heme, an entity that can be generated by UVA irradiation of cells, also appears to be a critical intermediate that can directly influence both the transcriptional activation and repression of the HO-1 gene. Heme release is likely to be of central importance to the inflammatory response in skin and its abrogation by HO.