Previous studies have demonstrated that phototherapy depresses serum riboflavin in jaundiced infants. The potential long-term hazards of thisin vivoreaction may be significant in view of thein vitroreaction of riboflavin which modifies intracellular DNA in eukaryotic and prokaryotic cells. Previous investigations have suggested that the DNA-modifying activity of riboflavin results from the generation of singlet oxygen and photooxidation of the guanine moieties of the DNA. In the present study, we demonstrate that singlet oxygen is not involved in the photodynamic reaction of riboflavin and deoxyguanosine.SpeculationIn view of the known relationship between DNA-modifying activity and mutagenesis and carcinogenesis, the photodynamic reaction between riboflavin and deoxyguanosine may have adverse long-term effects on children exposed to phototherapy. A better understanding of the characteristics of this photochemical reaction may permit the development of a phototherapy unit effective in the photoisomerization of bilirubin yet devoid of DNA-modifying activity.