In experiments on (CBA X C57Bl)F1 and BALB/c mice it was shown that with the spleen mass increasing, due to exo- and endorepopulation, during the postirradiation regeneration of the spleen, the number of splenic colonies also increases to a certain level. When the spleen mass reaches 1/3 of that lost after irradiation the number of discrete colonies decreases. When the spleen mass is restored to about 1/2 solitary colonies do not form at all. The spleen mass can considerably exceed the normal level in conditions of intensive repopulation.