Concentrations of bilirubin in the plasma of 85 premature infants weighing less than 2000 gm at birth were measured daily for the first 6 days of life. These measurements are presented and discussed. Nine cases of kernicterus which developed with relatively low concentrations of plasma bilirubin are reported. Laboratory studies are cited to support the hypothesis that the development of kernicterus in these infants was enhanced by the use of the antibacterial combination of penicillin and Gantrisin®. Attention is called to the effect which antibacterial therapy may have upon the concentration of bilirubin in the plasma of premature infants and to the lower values for plasma bilirubin which occur in dying infants.