SEVERE jaundice in the neonatal period has come to be recognized as one of the major problems of that crucial time of life. Whether caused by erythroblastosis fetalis (due to ABO, Rh and its subtypes, or some other blood-group factor) or an infectious disease, or even if it is simply severe "physiologic" icterus, marked jaundice may be accompanied by damage to the brain. This brain damage (kernicterus) is believed to be caused by high concentrations of indirect bilirubin. It is considered preventable by exchange transfusions given in time, and serum bilirubin levels are considered a reliable guide to the necessity . . .