Reactions due t o the administration of blood contaminated by Gram-negative bacilli are among the most severe risks of blood transfusions. The contaminating microorganisms more commonly responsible for these reactions, Aerobacter, Pseudomonas, Escherichia and Paracolon [l, 4, 11, 13, 141, share the following characteristics: production of very active toxins, ability t o grow at low temperatures and greater resistance than other germs t o the bactericidal effect of blood [l, 2, 3, 51.The purpose of this paper is t o report five such reactions in which massive bleeding was a constant symptom and a prominent factor in the death of three of the patients. Thrombolytic activity was found in a single case with adequate coagulation studies.Description of the reactions. Information concerning age, sex, diagnosis of the primary disease, type of surgical procedure and total amount of transfused blood are given in table I.The reactions were observed in a hospital for pulmonary diseases in a period of one month; they started during surgery (cases 1 , 3 and 5) or shortly thereafter. Death supervened within 3 hours in cases 1 to 3, and on the third day in case 4; patient no. 5 recovered. All patients were under general anesthesia and were receiving a blood transfusion at the time reactions began.Massive generalized bleeding and profound hypotension were the most prominent symptoms. Both lasted until death in patients 1 t o 3;in cases 4 and 5 bleeding stopped after 2 and 8 hours, whereas the hypotensive stage lasted until death in case 4 and for 10 days in the surviving patient. Cyanosis was also a constant and early symptom.Cases (4 and 5) surviving longer, had frequent emesis, profuse perspiration and great restlessness after recovering from anesthesia.