The total number of nucleated cells in the bone marrow of SAS/4 mice increase some twofold between 1 and 24 months of age but when related to body weight remains essentially constant over a wide range of ages. The concentration of CFU-S in femoral marrow is also constant with age and since other bones containing marrow appear, at least in young mice, to have the same CFU-S concentration as the femur it is concluded that the CFU-S compartment size of the whole bone marrow is independent of age when expressed on a body weight basis, In contrast, both the absolute number and the concentration of exogenous CFU-S in the spleen decline markedly in old mice. Smilary there is a decline in the number of endogenous colony-forming cells and the spleens of 24-month-old mice seem virtually devoid of such colonies. Not only were older mice less capable of supporting the growth of endogenous colonies, but their spleens also appear to provide a poorer environment for exogenous colony growth when compared with growth in younger recipient spleens.