BALB: Effects of DR and age on in vivo functional abilities of BMCs and numbers of HSCs. BMCs from young AL, young DR, old AL, and old DR, female BALB (Gpi1^a) mice were each mixed 1:1 with BMCs from a standard competitor pool of young female congenic BALB (Gpi1^b) mice. This mixture was injected intravenously into lethally irradiated young female BALB recipients. Donor (Gpi1^a) contributions to erythrocytes and lymphocytes in the peripheral blood of replicates were averaged 6 months after transplantation. A total of 3-9 young AL, old AL, or old DR donors were used over the course of 3 experiments. (A) Total marrow functional ability based on the average composition of erythrocytes and lymphocytes (E & L). (B) Total marrow functional ability expressed in repopulating units. (C) Numbers of LT-HSCs present in the original donor marrow using the mKSL flow criterion. Counts from young DR were not available (**P < .01; ***P < .001). (D) Functional ability per LT-HSC in the subset of animals using mKSL flow criterion (*P < .05; **P < .01). (E) Numbers of HSCs present in the original donor marrow using the SP + Kit flow criterion. Counts from young DR were not available. (F) Functional ability per HSC in the subset of animals using the SP + Kit flow criterion (**P < .01). Functional ability of old AL versus others in panel A is significant (P < .001). Error bars represent SE.