Blood group glycosyltransferases (A and B) in plasma are mainly derived from other tissues, not from bone marrow where the blood group substances are synthesized. Therefore, it is possible to determine the original blood type of host subject and grafted cells by examining the enzyme activity in plasma from a chimera subject. Subject WD had 75–85% of·type cells and 15–25% of A1 type cells. A1 enzyme activity of the subject’s plasma was the same as control A1plasma, suggesting that subject WD is genetically A1O1or A1A1and·red cells are produced by OO bone marrow cells acquired in utero. Another subject EA has about 85% of B type and 15% of A1B type red cells. B enzyme activity of the subject’s plasma is normal, but A enzyme activity is only 20% of normal level. Subject EA must be genetically BO or BB, and A gene is restricted to bone marrow cells acquired.