Recent investigations have shown that haemolysis plays a major role in the etiology of transitory hyperbilirubinaemia of the newborn. Our studies indicate that unconjugated bilirubin itself is in part the causative agent for this haemolysis. In vitro bilirubin causes damage of red cells which finally results in haemolysis. Newborns suffering from transitory hyperbilirubinaemia have a red cell population which is especially sensitive to the haemolytic effects of bilirubin. The initial bilirubin increase after birth is caused by relative dysfunction of the liver, causing further haemolysis of bilirubin sensitive red cells. Hyperbilirubinaemia decreases when all sensible red cells are destroyed and liver function improves.