Cytogenetic and biochemical studies demonstrate that the majority of human neoplasms consists of clones of mutant cells (1-5). In leukemia as well as in certain related conditions such as polycythemia vera and myelofibrosis, the bone marrow can often be shown to be populated by proliferating tissue, the dividing cells of which essentially all bear the same "marker" chromosome rearrangement. When leukemia is brought into clinical remission, normal cells, recognized by their normal chromosome complement, may reappear in the proliferating marrow (6). Their representation then among the dividing cells constitutes evidence that nonneoplastic hemic stem cells (7-9) survived throughout the more florid stage of the disease, but were not proliferating actively. They were not in evidence earlier because of the overpopulation of the marrow by mutant cells, being either suppressed or just not evoked into celldivision cycles. Their presence in a marrow signals hope for recovery of a normal marrow provided the situation is manipulated suitably. The manipulation proposed here consists of several steps, each of which would demand great adroitness and cooperation on the parts of the clinician and the cytologist. In broad terms the procedure would be: (a) aspiration of diseased marrow from a patient whose leukemic cells have in their chromosome complement a detectable rearrangement. (b) Massive culture in vitro of the mixed population of stem cells, many of which will be mutant, but others normal. (c) Establishment of a large number of clones. (d) Screening for normal hemic stem-cell clones, i.e. those both exhibiting a normal chromosome complement (and therefore not derived from cells of the leukemic clone) and being demonstrably pluripotent (capable of differentiating into progenitors of the various differentiated cells of the marrow. (e) Expansion in vitro of these clones of normal pluripotential stem cells, employing culture conditions that encourage proliferation but not differentiation. (f) Repopulation of the patient's marrow with the culture of his own pluripotential stem cells after his diseased marrow has been ablated by standard radioand chemotherapeutic