The stored blood employed during World War II for the treatment of battle casualties was almost exclusively group-O blood, in consequence of which patients of blood groups A, B and AB were obliged to receive incompatible isoagglutinins in the course of transfusion. Anti-A and anti-B isoagglutinins, incompletely absorbed in the process of pooling, were likewise contained in the dried plasma supplied by the Army. Thus, all patients of blood groups other than group 0 receiving pooled plasma, and over 50 per cent of those who were transfused with whole blood were subjected to the injection of incompatible isoagglutinins.The preponderance of evidence in the literature indicates that the transfusion of group-O blood into recipients of other blood groups rarely produces serious reactions (1).