A new blood factor present in 3000 random bloods tested was descr. with the aid of a hemolysin produced by a 66-year-old patient suffering from gastric carcinoma. The antibody failed to react with the red cells of the patient''s sister, but it reacted with the blood of her other siblings and their offspring. The main interest in this case lies in the origin of an apparently immune antibody. Since pregnancies and transfusions could be excluded, it is assumed that the patient was immunized by the presence of the antigen in her tumor cells, detectable by specific absorption. The assumption is made that the blood factor in the tumor cells arose by mutation.