The use of blood belonging to group O as "universal blood" has been practiced for many years. With the rapid increase of the number of transfusions, it has become evident that the transfusion of universal blood is not as safe a procedure as was originally believed. The blood cells of group O cannot be responsible for reactions following a transfusion of universal blood because, with rare exceptions, they are not agglutinated by any kind of human serum at body temperature. The reactions obtained are attributed by many authors to the blood plasma of the donor containing the isoagglutinins anti-A (a) and anti-B (β). In the majority of cases the titer of these agglutinins is not high enough to cause trouble, but in some instances the titer may be high or even extremely high. Cases have been reported in which there are titers occasionally reaching 500 and more. Justification for the