Accidents following transfusion have been sufficiently frequent to make many medical men hesitate to advise transfusion, except in desperate cases. It has been our opinion since we began making observations on this question in 1908 that such accidents could be prevented by careful preliminary tests, leading to the exclusion of agglutinative or hemolytic donors. Our observations on over 125 cases have confirmed this view and we believe that untoward symptoms can be prevented with absolute certainty. It has been the general impression that preliminary blood-tests ought to be done to determine the compatability of the donor's with the patient's blood, but there has been little direct observation as to the correspondence between the test-tube phenomena of agglutination and hemolysis and symptoms arising after transfusion. Scattered reference to the hemoglobinuria following transfusion have made it clear that the occurrence of hemolysis between the donor's and patient's blood is a real danger.