The case records of 11 infants with prolonged hyperbilirubinemia have been presented. The elevation of indirect reacting bilirubin in these infants may be due to interference with the normal conjugating mechanism by a substance present in breast milk. Introduction of cow's milk formulas should be carried out as part of any extensive investigation of prolonged hyperbilirubinemia of the indirect type in otherwise well infants who have been wholly breast fed from birth.