ABO hemolytic disease of the newborn without hyperbilirubinemia is described in 17 full-term infants. The erythrocyte characteristics, such as reticulocytosis, microspherocytosis, and positive indirect antiglobulin (Coomb's) test, resembled those in ABO disease with hyperbilirubinemia. Erythrocyte acetylcholinesterase activity was reduced in this group to the same degree as in the more severely affected infants. Negro infants predominated over white, in a 2.5:1 ratio in this mild ABO group.