Several endogenous cellular constituents were tested for their ability to produce superoxide anion (O2-) from ground-state molecular oxygen upon irradiation by solar radiation. The pyridine cofactors NADPH and NADH, riboflavin, and the nucleosides 2-thiouracil and 4-thiouridine were found to sensitize the transmission of photon energy from solar radiation and monochromatic radiation (290, 334, 365, and 405 nm) to oxygen, resulting in O2- formation, as detected by superoxide dismutase-inhibitable cytochrome c reduction. Quantum yields for the production of O2- indicate that NADPH is the most efficient and riboflavin the least efficient of the compounds tested. These data indicate that endogenous compounds may participate in the production of O2- by solar radiation and imply that O2- may play a role in sunlight-induced erythema and dermal carcinogenesis.