The purpose of this article is to describe a blood factor apparently not encountered previously. A case is reported in which sensitization to the new blood factor caused a fatal hemolytic reaction. REPORT OF CASE A Negroid woman, aged 35, was admitted to a hospital in Manhattan with the diagnosis of a bleeding peptic ulcer. According to the information received, the patient had been pregnant three times but had only one living child. On admission a blood count showed severe anemia, 4 gm. of hemoglobin per 100 cc, 1,280,000 red blood cells, and 29,000 white blood cells, with 73% neutrophils, 6% metamyelocytes, and 20% lymphocytes. A blood transfusion was attempted but discontinued after 100 cc. had been administered, because of a reaction consisting of chills and fever. A week later a blood count showed persistence of the anemia, and another From the Serological Laboratory of the Office of the Chief