A possible source of trouble will occur to anyone looking critically at the table of reactions, for it will be noticed that the serum of Group IV, the so-called “universal donors,” agglutinates the corpuscles of all the other groups. How does it come about, therefore, that the blood of this group may be given indiscriminately? The answer is to be found in the fact that though the reaction takes place as shown in the table _outside the body_, nevertheless the serum of the transfused blood does not exert its agglutinating power in the body of the recipient. Several hypotheses have been advanced to account for this discrepancy, though no final explanation has yet been arrived at. In the first place it is possible that the agglutinating power of the serum is rendered ineffective by the dilution which it undergoes when it is mixed with the blood of the recipient. It has been shown, however, by Culpepper that agglutination takes place outside the body with serum diluted up to 1 : 150, a degree of dilution far greater than is ever obtained in a transfusion where the dilution in the patient’s circulation is usually no greater than 1 : 7. Secondly, it has been suggested that the transfused plasma meets with an excess of plasma containing protective or antihæmolytic properties. The evidence on this point is conflicting. Hektoen in 1907 was unable to demonstrate any such property in serum or plasma. Brem and Minot in 1916 both claimed to have demonstrated antihæmolytic properties in serum, and Minot added the observation that its concentration varies. Karsner in 1921 reported that he had failed to demonstrate anti-agglutinins in the blood. For the present, therefore, the point must remain undecided. Finally, it is possible that the agglutinins of the transfused plasma, meeting with an excess of agglutinable cells, are all absorbed without actually producing any agglutination. Whichever of these hypotheses be true, the fact remains that the blood of Group IV individuals may be given without serious effects in most ordinary cases in which transfusion is indicated.