An assay system for the number of viable cells in suspensions of transplanted AKR/J lymphoma cells, based on the enumeration of colonies in the spleens of nonirradiated recipients, is described. This method has been used to examine the radiation sensitivity of these cells and has yielded a survival curve with D. of 114[plus or minus]4 rads and an extrapolation number for cells irradiated in vitro with Co 60 -rays of 0.71 [plus or minus] 0.16. The data presented suggest strongly that the spleen-colonies used in the assay are, in fact, clones of malignant cells.