The fractions of protein in cerebrospinal fluid of 10 infants with hemolytic disease of the newborn, and 12 newborn infants with other illnesses, have been analyzed by paper electrophoresis. Relative concentrations have been found to be almost constant, despite wide individual variations in total concentration of protein—except in inflammatory disease with involvement of the central nervous system. Relative concentrations of gamma globulin are higher in serum than in cerebrospinal fluid, while concentrations of beta globulin are greater in cerebrospinal fluid. It is suggested that the total concentration of protein in cerebrospinal fluid in newborn infants may reflect the degree of functional maturity of the blood-brain barrier system.