1. Twenty-four full-term, newborn infants, of whom 23 were Rh-isoimmunized, were followed with serial determinations of the serum and spinal fluid bilirubin and spinal fluid total protein during the neonatal period. Of the 51 spinal fluid samples examined, measurable quantities of bilirubin were present in all cases. 2. In the presence of marked indirect hyperbilirubinemia, the spinal fluid bilirubin was predominantly indirect-reacting, though no linear relationship exists between the serum and spinal fluid bilirubin. It is suggested that this is a manifestation of individual variations in blood-brain barrier permeability within the neonatal period. A significant correlation between the spinal fluid indirect bilirubin and the spinal fluid total protein is further evidence to support this view. 3. Spectrophotometric absorption studies of 13 serum and spinal fluid samples from 8 infants with hemolytic disease of the newborn are discussed. These studies tend to confirm the observation that there is no direct correlation between the hyperbilirubinemia and the spinal fluid bilirubin concentration, though the configuration of the absorption pattern of the serum closely follows that obtained from the corresponding spinal fluid. 4. The findings in the Rh-isoimmunized group of infants were not unlike those observed in a single case of "physiologic jaundice" presented. 5. The question of indirect bilirubin neurotoxicity is briefly reviewed.