In the late 1940s investigators observed that mice given supralethai doses of total body irradiation were protected by infusion of viable spleen or marrow cells following irradiation, and that this was accomplished by hemopoietic reconsti tution with donor cells as proven using genetic markers. If a similar approach could be applied to humans, it should be possible to treat leukemia patients with any dose of chemoradiotherapy as far as nonmarrow toxicity permitted, and then rescue them by marrow transplantation. Early clinical attempts were gen erally unsuccessful, mostly due to a lack of...