Abstract
The numbers of CFUโS which developed in spleen colonies were measured 11 days after injection of irradiated mice with marrow from normal mice or mice which had been treated in one of a variety of ways. The broad spread of CFUโS numbers, seen by other authors, in colonies derived from normal marrow was confirmed. However, the range and distribution of CFUโS per colony was generally different in colonies derived from the marrow of mice which were recovering or had recovered from some form of depopulation. From the data obtained, the mean CFUโS/colony, M^1^, and the probability of selfโrenewal, p, of the CFUโS were calculated. These values are used to calculate the number of cell cycles undergone during development of the colony and, by making certain assumptions, the cell cycle time of the CFUโS. The plot of p against log M for the various samples measured should be linear if all CFUโS proliferate at the same rate in a growing colony. It is not linear, however, so that CFUโS obtained under different experimental conditions do not all undergo the same number of cycles. In general, treatments given to the mice result in a lowering of the capacity for selfโrenewal of their CFUโS and also to a shortening of their cell cycle time. Some of the possible implications of these findings are discussed.