HLA antibody formation was studied in 212 patients undergoing open-heart surgery on cardiopulmonary bypass in relation to the following variables: sex, obstetric history, transfusion history, quantity and nature of blood products transfused, type of oxygenator used, duration of bypass, and time of assay. Postoperative alloimmunization was seen in 33.6 percent of males and 64.3 percent of females. Previous pregnancy seemed to favor the development of antibody. No statistically significant correlation was noted between alloimmunization and the amount of blood products transfused. The best time for assay was day eight following surgery.