Attempts to transfer bone marrow met with little success before the 1940's. It had still to be realized, that bone marrow differentiated from transplantable stem ceils, which engraft only in recipients needing stem cells. Also, the importance of histocompatibility between donor and recipient was not recognized. One thing that was puzzling, due to its simplicity rather than its complexity, was the method of administering bone marrow: Earlier applications of bone marrow per os, by intramedullary or intraperitoneal injections were finally replaced by the intravenous route (for historical review see [1], [10]). In the 1950's, when the fear of nuclear warfare had mobilized biologists in the United States to study systematically the effects of whole body irradiation, Loren z et al. [2] reported in 1951 that lethally irradiated mice could be saved by the intravenous injection of bone marrow from other mice, and not only by shielding the spleen of the irradiated mice as Jacobsen et al. [5] had demonstrated shortly before. Experimental data by Barnes et al. from Harwell in England [4] supported the "cellular hypothesis" arising from Loren z' observation of a cell-born repopulation of the bone marrow cavity. In contrast the "humoral hypothesis" postulated factors in cell-free homogenates of spleen or bone-inducing hemopoietic regeneration after irradiation [3], [5]. The final confirmation of successful bone marrow transplantation was the proof in 1956 of a persistence in the marrow recipient of mouse [6] or rat [7,8] donor cells, which could be distinguished by cytogenetic, histochemical or serological techniques. When highly inbred strains of mice became available it was recognized that bone marrow could be easily grafted only within the same strain. Experiments with irradiated mice which had been given marrow from foreign, histoincompatible strains [9,11,13] remained less successful. While the laws of transplantation were explored on the basis of skinor tumor graft rejection by the host, researchers attempting bone marrow transplantations realized that they were dealing additionally with the reverse situation: an immunocompetent graft "rejecting" its host.