Standard phototherapy illumination of human cells in the presence of riboflavin is known to produce single-strand breaks in intracellular DNA. A new photochemical reaction is described between riboflavin and purified DNA in which an adduct is formed. Unlike the previously described oxygen-dependent reaction between light-activated riboflavin and deoxyguanosine, this new photochemical reaction is oxygen-independent and involves deoxyadenosine or thymidine.