1. The isoagglutination reaction of 131 infants and children from birth to 10½ years was examined by testing their serum and washed corpuscles microscopically against the serum and corpuscles of each of the four adult groups.
2. The grouping as present in adults is rarely present in blood from the umbilical cord.
3. At birth and during the 1st month of life isoagglutination is rarely present, but the percentage of infants in whom the isoagglutinin group is established increases with age, so that after 1 year the group is usually established, and after 2 years is always present as in adults.
4. The grouping is established in the corpuscles before it is established in the serum; i.e., the corpuscles acquire agglutinophilic receptors before the serum acquires agglutinin. Therefore, Group I is the first group to be formed and Group IV is the last.
5. The early grouping in the corpuscles before the group is established in the serum is liable to change by the acquisition of new receptors.
6. When the grouping has been established in both serum and corpuscles it does not change.
7. Isoagglutinins are present in mother's milk and the grouping is identical with that in the mother's blood. These agglutinins are probably not transmitted to the nursing infant through the milk.
8. On account of the differences between the agglutination reactions in the blood of mother and child it is not safe to transfuse an infant from its mother without making the preliminary tests.