Erythroblastosis fetalis is an important cause of fetal death and of cerebral palsy. As long as the baby is in the uterus, anemia is the serious threat to life, but after birth jaundice is the important cause of death or of crippling. Jaundice can be controlled and brain damage can be prevented by exchange transfusions given early and repeated if necessary. Babies who do not become too anemic in utero should now recover in all cases if proper treatment is given. The important problem remaining to be solved is that of intrauterine death caused by failure of the fetal hematopoietic system to keep up with the abnormally rapid destruction of red blood cells. The fundamental cause of erythroblastosis fetalis is incompatibility between fetus and mother with respect to one of the blood group factors (the Rh factor in the typical case). When the mother lacks a blood factor that is