Within the past five years, two new blood factors, U and V, have been discovered during blood transfusions in patients with anemia. The first, blood factor U, was found by Wiener, Unger, and Gordon1 in a Negro woman with a diagnosis of bleeding peptic ulcer, a history of three pregnancies, and severe anemia. The first blood transfusion was discontinued after the administration of 100 ml. of blood because of chills and fever in the patient. A week later, a second blood transfusion was started that also had to be discontinued, as the patient went into shock and later died. This fatal hemolytic transfusion reaction was due to the sensitization of the patient to the new blood factor, U. The blood factor was designated as U to indicate its almost universal distribution in the population. Wiener, Unger, and Cohen,2 in another study, found that this blood factor occurred in