The number of viable cells remaining in any sample of irradiated cells i mined by finding how many are capable of continued proliferation. Two for accomplishing this have been described, and both are limited in their appl The first method (1) depends on irradiating cells in tissue culture and fin many can subsequently form clones; this method is open to the objection may behave differently in culture from the way they do in the animal. T method (2) is limited to tumor cells and depends on ascertaining how m of any irradiated sample are capable of developing into tumors after transplan into new hosts. Neither method permits the determination of the viabi example, of the cells of normal bone marrow growing in their normal situati irradiation.

The method described in this paper for determining the number of viable cells in bone marrow irradiated in vivo and in vitro hinges on the fact that marrow cells from normal mice, injected into lethally irradiated mice, multiply and differentiate within the hemopoietic organs of the recipients so as to prevent their death from marrow failure (3-6). Thus the survival of an irradiated mouse depends on the viability of cells in the transplanted marrow. Further, it has been shown that the percentage survival of irradiated animals receiving marrow injections is a function of the total number of cells injected (7, 8). Therefore, the percentage survival of a group of irradiated mice injected with bone marrow can be used to measure the number of viable cells in the suspension used for injection. In the experiments