--Disease Characteristics-- Hyperbilirubinemia associated with either of the following: Direct Coombs' test-positive ABO hemolytic disease of the newborn Glucose-6-phosphate dehydrogenase deficiency --Prior/Concurrent Therapy-- No maternal phenobarbital in last month of pregnancy --Patient Characteristics-- Performance status: Not specified Hematopoietic: Not specified Hepatic: Not specified Renal: No congenital renal abnormality Cardiovascular: No congenital heart abnormality Pulmonary: No asphyxia requiring assisted ventilation at delivery Other: Gestational age more than 210 days Birth weight at least 1500 g No other major congenital abnormality, i.e.: Central nervous system Chromosomal Gastrointestinal No evident or suspected congenital infection, e.g.: Cytomegalovirus Herpes Rubella Syphilis