Exposure of normal human fibroblasts to visible light (420-490 nm) in the presence of exogenously added 1-100 μg/ml bilirubin enhanced the level of DNA strand breakage compared with cells irradiated in the absence of added bilirubin. Treatment of cells in the dark with an irradiated bilirubin solution also induced DNA strand breaks. However, strand breakage was not detected in cells treated with an irradiated bilirubin solution that had been incubated with catalase (H2O2: H2O2 oxidoreductase EC 188.8.131.52). Examination of irradiated bilirubin solutions demonstrated the presence of hydrogen peroxide although, apparently, not at concentrations sufficient to account for the level of DNA strand breakage detected. Hence, irradiation of bilirubin results in the generation of hydrogen peroxide and possibly other peroxides that can cause DNA damage.