Homotransplants are transplants between genetically dissimilar individ uals or, more specifically, from one inbred strain to another unrelated in bred strain. Isotransplants are transplants within a genetically uniform in bred strain. Typically, isotransplants will grow, but homotransplants will not. The consistency of this behavior of transplants within, as compared with between, inbred strains has made such strains, and particularly the large variety of highly inbred strains of mice, one of the basic tools of investigation in this area. Transplantable tumors, because of the ease and rapidity with which subcutaneous or intra-abdominal transplants can be performed, are also a valuable research tool, though normal tissue grafts are the preferred instrument for many purposes.