"In a previous publication I showed that the transfused red blood-corpuscle does not have a transitory existence in the body, but that it remains in the blood-stream for a considerable length of time. The method used to show this is dependent on the iso-agglutinins present in blood and is based on the fact (which demonstrated with in vitro mixtures of blood corpuscles) that corpuscles belonging to two different blood groups if mixed can be separated quantitatively from one another by treating with a serum that will differentially agglutinate the corpuscles of one kind, leaving the others free. This method applies equally well to in vivo mixtures; the transfused blood-corpuscles in a patient can be separated from. the recipient’s native corpuscles by differentially agglutinating the recipient’s corpuscles with appropriate serum, provided the donor and recipient are in different groups. This method is only applicable, of course, to cases in which patients belonging to (’s nomenclature) are transfused with Group IV blood, or in which patients belonging to Group I are transfused with Group II or Group III blood, but in these cases the history of the transfused blood can be followed readily."