A series of 1,704 infants of blood group O mothers have been studied to determine the relation between the degree of red cell sensitization and the cord hemoglobin and bilirubin concentrations. The infants with blood group A or B had significantly higher cord bilirubin and lower cord hemoglobin concentrations than the group O babies. Those infants whose red cells had the greatest evidence of sensitization had the highest bilirubin and lowest hemoglobin levels. The infants in whom no antibody was demonstrable on the red cells or in the red cell eluate also had significantly higher cord bilirubin and lower cord hemoglobin levels than the ABO compatible group; it is suggested that these infants had sufficient erythrocyte sensitization to produce mild hemolysis. ABO incompatibility represents a spectrum of hemolytic disease extending from those in which there is little laboratory evidence of erythrocyte sensitization, but evidence of hemolysis, to severe hemolytic disease in which erythrocyte sensitization is usually easily demonstrable.