The protection afforded to man against epidemic diseases through the employment of prophylactic antiserum injection is of course best illustrated by the case of diphtheria. It is likewise known that many infections to which animals are subject can be prevented by a similar form of passive immunization. While the number of experimental infections which can be prevented in this manner is not large, it is a notable fact that the blood of animals which have recovered from hog cholera carries a protective principle capable of warding off, for a time, infection with the filterable virus of that disease. That human blood of one individual can be injected safely into another individual is of course a commonplace. Recent experience with the transfusion of large quantities of blood is convincing in establishing the value and safety of this procedure. Experience has also shown that human blood carrying immunity principles can be employed