Studies were made of the blood clotting system of 38 patients who received massive transfusions in connection with cardiac surgery. Of 30 patients who underwent openheart surgery, 19 received from 3,000 to 11,500 cc. of blood and did not bleed, while 8 received from 4,000 to 21,000 cc. of blood and did bleed. Eight patients who underwent closed-heart surgery, and had hemorrhagic episodes subsequently, received from 3,100 to 15,000 cc. of blood. It appeared that the hemorrhages observed were more probably the occasion rather than the result of the massive transfusions. Eight quantitative laboratory tests were applied to blood samples from these patients, before, during, and after the surgery, but no common cause of bleeding was evident. There was no significant difference between bleeders and nonbleeders with respect to platelet count, fibrinogen concentration, or status of the plasma coagulation system either preoperatively or postoperatively. It would appear that massive transfusion aggravates a tendency for "unexplained bleeding in the surgical patient," but the primary blame for hemorrhage cannot be placed on compatible massive transfusions.