Photosensitizers such as methylene blue or riboflavin enhance the rate of bilirubin photodegradation in vitro. This effect is studied with respect to dose-response and time-response relationships, the influence of concentrations of bilirubin and albumin, and the oxygen consumption. The results are discussed in view of the probability that the effects described also occur in vivo and that riboflavin supplementation to neonates could improve the phototherapy of neonatal hyperbilirubinaemia.