IT SEEMS well established that there is a relationship between breast feeding and neonatal hyperbilirubinemia.1-5 Transient unconjugated hyperbilirubinemia observed for several days in the neonatal period, and clinically referred to as physiologic jaundice, is believed to be the result of a maturational delay in the development of the hepatic enzymes associated with bilirubin conjugation in the liver.6-8 Both pregnane-3α, 20α- and pregnane-3α,20β-diol show in vitro inhibition of conjugation in a system using o-aminophenol and bilirubin as substrates with microsomal fractions prepared from rat and guinea pig liver.9-11 An in vitro inhibition is also demonstrable with breast milk of some nursing infants with so-called physiologic hyperbilirubinemia in the same system.12 Arias et al have reported the isolation of the 3α,20β isomer from breast milk.2 They have estimated that approximately 1 mg of the substance is excreted per day in the mother's milk. Two full-term infants orally