To the Editor.
—In view of the popularity currently enjoyed by the cell kill hypothesis in the therapy of malignant disease, I applaud the questioning editorial by Weintraub, "Do Remissions in Acute Leukemia Depend Upon 'Cell Kill'?" (Arch Intern Med 128:498-499, 1972).Acute leukemia unquestionably represents a failure of cellular differentiation and maturation. Controversy surrounds the reasons for this failure. The clonal theory views leukemia as replacement of the marrow by the progeny of a cell inherently incapable of normal maturation. The cell kill rationale of therapy is a logical outgrowth of this theory. An alternate explanation is that the disease is the result of a deleterious effect on intrinsically normal cells by external and potentially reversible influences, as stated by Boggs.1 The recent report of development of leukemia in donor cells after marrow engraftment2 gives impetus to the external agent theory, while striking a severe blow at