Till, McCulloch and Becker^1,2 have demonstrated that normal mouse hæmatopoietic tissue contains a class of cells which are characterized by their ability individually to give rise to macroscopic colonies in the spleens of heavily irradiated recipient mice. The ease with which the colonies may be enumerated has permitted quantitative investigations of the effects of irradiation^3,4 transplantation⁵, and erythropoiesis⁶ on the number of cells with colony-forming potentiality. These varied applications suggested to us that the spleen-colony technique might be fruitfully applied to the examination of malignant tissue. The results show that a quantitative assay of the number of murine lymphoma cells capable of proliferating in vivo is, indeed, possible.