THE blood-group antigen Rh, present in the red blood cells of about 85 per cent of normal people, called Rh-positive, and absent in the rest, Rh-negative, plays, with its corresponding anti-Rh agglutinin, an important part in the causation of erythroblastosis fœtalis, a familial disease of the new-born. In about 90 per cent of cases of the disease the mother is Rh-negative and has made anti-Rh which passes through the placenta and damages the red blood cells of her Rh-positive fœtus. The Rh factor is inherited as a dominant character with three genotypes RhRh, Rhrh and rhrh.