Three examples of an antibody were found to be detecting a red cell polymorphism probably identical to MER2. The antibodies were made by Jews originating from India and living in Israel. Two of them were sibs and the third was unrelated. All 3 had kidney disease requiring renal dialysis and regular blood transfusion. In 2 cases the antibodies were detected before dialysis was started and before the patients had been transfused. The human antibodies reacted with red cells of 90% of Israeli blood donors tested. In tests on selected blood donors, 82 English and 56 Israeli, one of the human antibodies gave almost identical reactions to those given by monoclonal anti-MER2. Anomalous reactions were probably due to anti-Bg^a. Two of the human antibodies completely blocked, and one partially blocked, the reaction of monoclonal anti-MER2 with MER2+ red cells.

A number of Israeli donors had anti-M in their sera, a proportion of which cross-reacted with N He(+) red cells. Anti-M^e was detected and while the reactivity with M and He determinants could be separated by using trypsin-treated red cells, the cross-reactivity for M and He determinants was complete in absorption experiments. One serum had anti-M separable from anti-M^e and another apparent anti-M was absorbed by trypsintreated N He(+) red cells.