The growth capacity of femoral bone marrow stem cells from young and old long-lived mice was assessed in the spleen of X-irradiated young and old syngeneic recpients by determining: (a) the number of stem cells colonizing the spleen, (b) the rate of incorporation of 125I-labeled iododeoxyuridine by proliferating colony cells, and (c) the number of cells present in the largest colonies at the end of the growth phase.We found that the growth capacity of stem cells declined with age. We further found that the spleen-seeking and spleen colony growth capacities of old stem cells remained characteristically old even after they were allowed to self-replicate in the bone marrow of young recipients for an extended period of time. On the other hand, the spleen colony growth capacity of young stem cells could be reduced by allowing them to self-replicate in old recipients. These results suggest that the growth capacity of old stem cells is an intrinsic characteristic which cannot be readily altered, but that of young stem cells can be aged in an accelerated manner by allowing them to self-replicate in old recipients. An additional reduction was noted in the frequency of both young and old stem cells colonizing the spleen of old recipients and in the cell density of the largest colonies produced. These results indicate that factors extrinsic to the stem cells are also responsible for the decline with age in their spleen colony growth capacity.Thus, the growth capacity of old stem cells in old recipients could be as low as 10% that of young stem cells in young recipients.