1. Severe hemolytic reactions were observed in 3 group A (subgroup A1) recipients transfused with group O whole blood or plasma. In one case, 10 ml. of a commerical preparation of soluble A and B factors had been added to 500 ml. of whole blood prior to the transfusion and it is believed that the reaction might have been even more serious had not this material been added.

2. The anti-A antibodies in the serum of the dangerous universal donors causing the hemolytic reactions fixed complement, acted as hemolysins, were difficult to neutralize with soluble A and B factors, were capable of giving positive Coombs tests and their ability to agglutinate A cells was enhanced by the presence of normal human serum. These characteristics were similar to those observed in serum from donors known to be actively immunized against the A factor, but the stimulus for development of "immune" anti-A antibodies in the dangerous group O donors was not apparent.

3. Small amounts of immune A antibody were consistently demonstrated in 12 of 100 random group O sera which, after neutralization, produced indirect Coombs tests with A1 cells and agglutinated A1 cells suspended in compatible normal human serum.

4. Screening procedures for elimination of dangerous group O donors are discussed.

ACKNOWLEDGMENTS It is a pleasure to thank Mrs. Jane Peters, Mrs. Nieves Dole and Miss Jean Dorothy for technical assistance and Dr. R. Wendell Davis for collaboration in the study of Case 1.