ANTI-TUMOUR therapy upon experimental animals has long served as a model for attempts to treat cancer in man. As with few exceptions these agents have marrow depressant qualities there has always been a serious danger that therapeutically effective doses will prove lethal as a consequence of haematopoietic failure. Within the last 10 years some encouragement has come from the demonstration that an injection of viable bone marrow can restore haematopoietic function (Lorenz, Congdon and Uphoff, 1952) when given after a dose of irradiation sufficient to cause haematopoietic failure but insufficient to damage irreparably the intestinal epithelium. It has been shown quite clearly that this restoration is due to repopulation of the depleted haematopoietic sites by the injected marrow (Ford et al., 1956). Barnes et al. (1956) and Barnes and Loutit (1957) were quick to realise the use of the finding to extend the doses of irradiation available as therapy against cancer. Others have adopted a similar approach though using radiomimetic chemicals. Although in both mice (Barnes and Loutit, 1957) and men (Thomas
