SummaryLittle information is available on the effect of visible light from commercial fluorescence lamps, commonly used in the treatment of neonatal hyperbilirubinemia, on parameters related to genetic damage in eucaryotic cells. The present study was undertaken to determine whether or not visible light of different wavelengths had any differential effects on the frequency of DNA breaks (frank breaks plus alkaline labile lesions), DNA replication, frequency of sister chromatid exchanges, and survival in cultured Chinese hamster cells.The results revealed that the “blue” spectral band (420 to 500 nm) is mainly responsible for DNA breaks, sister chromatid exchanges, and lethality induced by fluorescent light. This band is precisely that which bilirubin heavily absorbs and, hence, is the most efficient for the decomposition of this metabolite.These results were obtained with the use of light doses of 5 to 30 X 104J/m3versusthe light doses of the order of 100 X 104J/m2being received by infants undergoing phototherapy treatment.