Bone marrow cells from young and old mice (C3H/HeMs) were transplanted into irradiated syngeneic recipients. The growth rates of spleen colonies in both groups were compared by measuring the mean volume of colonies. Growth rates in spleen colonies derived from bone marrow cells of 462-day-old mice (Y-R) were higher than those from 917-day-old mice (O-R). Bone marrow cells from 62-day-old mice and those from 642-day-old mice were injected into 112-day-old irradiated (950 rad) recipients and after 400 days were killed. The bone marrow cells were assayed for spleen colony-forming units (CFU-S) and injected into irradiated secondary recipients, 50–55 days of age (Y-M-R and O-M-R). The cellular age at testing was 462 and 1042 days, respectively. The growth rates of colonies from young mice (Y-M-R) and from old ones (O-M-R) were similar in contrast to the first experiment in which the younger CFU-S produced more rapidly growing colonies. These studies clearly show that CFU-S from 462-day-old mice produce larger splenic colonies than CFU-S from 686- or 917-day-old mice. However, spleen colonies formed by CFU-S with cellular ages of 462 days (62 days + 400 days in vivo) and 1042 (642 days + 400 days in vivo) are not significantly different, suggesting that “in vivo incubation” has removed some of the proliferative defect of the 642-day-old CFU-S.