The Effects of Nitrogen Mustard and Dimethyl Myleran on Murine Leukemia Cell Lines ofDifferent Radiosensitivity in Vitro1

as “radiomimetic― because, like X-rays, these agents give rise to mitotic death (7) ; that is, dividing cells are “steri lized―in the sense that they are prevented from undergoing indefinite duplication, and an individual cell loses the ca pacity to form a clone. After sustaining a lethal injury, defined as an injury causing inability to form a clone, cells may divide several times and can persist and metabolize for long periods, during which they are recognized as living by physiologic tests such as dye uptake. To cause out right death, called interphase death (1 , 7), very much larger doses are required ; the mechanism involved is en tirely different, and the relative effectiveness of different substances is not the same for the 2 types of death. When drugs are used to treat tumors in vivo, the dose levels at tamable are such as to kill by the mitotic pathway (1, 7).