SUMMARY: Administration of oestriol, pregnanediol or cortisone to newborn infants increased the level of unconjugated bilirubin in the blood serum. Progesterone had no such effect. Administration of oestriol, progesterone, pregnanediol and cortisone to the nursing mother also raises the bilirubinaemia of the suckling infant, presumably by the excretion of metabolites in the milk. However, in therapeutic doses, anabolic steroids, tri-iodothyronine, prednisolone and dexamethasone, which are metabolized differently, do not increase serum bilirubin. The increase of bilirubin levels by naturally occurring steroids can be explained as an effect of competition between these compounds and bilirubin for the conjugating enzyme glucuronosyl transferase in the liver of the newborn. It is suggested that the large quantities of placental and foetal steroid hormones, which have to be conjugated and excreted by the newborn in the first days of life, are the cause of hyperbilirubinaemia and icterus neonatorum. The need to conjugate these steroids and bilirubin at the same time exceeds the capacity of the immature liver to produce sufficient amounts of the conjugating enzyme during the first 3 weeks of life. The finding that administration of conjugated steroids does not elevate serum bilirubin is in favour of this hypothesis.