Success in transplanting tissues in animals, particularly the lifesaving effect of transplants of marrow after lethal exposures to radiation, has awakened interest in the transplantation of human tissue. In general the tissues of one person do not flourish in the body of another. Genetic differences in the chromosomes of host and donor predetermine differences in their cellular constituents,1 presumably at the molecular level.2 These differences are sufficient to excite immunologic response.3 Except in the case of identical twins, a graft is recognized by the defense system of the recipient as possessing foreign antigens.4 The lymph centers react accordingly,5,6 and the transplanted tissue is subjected to round-cell infiltration and inflammatory change. In a week or two the graft is rejected. The invasion of the graft by lymphocytic cells is decisively important. Grafts protected against cellular invasion by semipermeable membranes remain alive until accumulations of fibrous tissue impair nutritive exchange.7 Transplanted