“…This method implies that a rise of serum bilirubin greater than 0 3 mg./hour in a mature baby (0 2 mg./hour at 36 weeks' gestation) was potentially dangerous. Although this is a considerably lower criterion than the rate of 0 5 mg./hour recommended as a guide by Walker (1959) and Adelman, Bell, Giunta, Barrett, Bellin and Appleton (1959), its use in this series was not reflected by a high 'exchange' rate. The value of this hypothesis was supported by the fact that only 3 % of the exchange transfusions were performed on babies more than 60 hours old.…”