The value and far-reaching implications of successful homotransplantation of tissues and organs in humans can hardly be over-emphasized; but up to the present, the transfer of tissues and organs from one individual to another, except in special instances (for example, in identical twins and inbred strains of animals, and in agammaglobulinemia), has been unsatisfactory. The primary obstacle to survival of homografts appears to be the destructive antigen-antibody reaction between the host and the foreign tissue. The mechanism of homograft rejection is the subject of many current investigations. An important study shedding some light on the problem is that of Algire and his associates.1 On the basis of the observation of Merwin and Hill2 that free homografts of thyroid or harderian glands of newborn mice survive without initiating immunity until vascularized by the host, they used millipore filter chambers in their study of the immune reaction induced by grafts.