Photodegradation of bilirubin in vivo has been investigated by using green fluorescent sources according to the suggestions obtained in a previous study in vitro. Two groups of 50 jaundiced low-birth-weight infants were exposed to fluorescent light in phototherapy units under similar irradiant conditions. One group was exposed for 24 h to standard green tubes, the other, to daylight lamps. Newborns of the two groups were similar for gestational age, birthweight, and initial bilirubin concentration. A greater decrease in the serum concentration of bilirubin was found in subjects exposed to the green light when compared with infants exposed to daylight lamps. The use of green light is strongly suggested instead of the white, blue, and special blue lamps, because of the real efficiency, power and range of wave lengths.