During phototherapy for neonatal jaundice, bilirubin is converted into a variety of photoproducts. Determination of the relative importance of these photoproducts to the elimination of bilirubin requires knowledge of their rates of excretion. We have measured the rate at which the structural isomer of bilirubin, lumirubin, disappeared from the serum of nine jaundiced premature infants after the cessation of phototherapy. In all patients studied, the decline in serum lumirubin could be approximated by a first-order rate equation with a half-life of 80 to 158 min. This rate of disappearance is much faster than that previously determined for the other major bilirubin photoproducts. In samples of bile aspirated from the duodenum of infants undergoing phototherapy, lumirubin was the principal bilirubin photoproduct found. These results indicate that formation and excretion of lumirubin is an important route for bilirubin elimination during phototherapy.