Aged BM is inefficient at producing T-lineage cells in vivo in nonirradiated recipients. (A) Lethally irradiated (9 Gy) Ly5^SJL recipients were intravenously injected with 1:1 mixtures (3 × 10⁵ cells each) of 18- to 24-month-old Ly5^B6 BM and 2-month-old competitor Ly5^SJL BM (▒). Control chimeras (░) were generated by injecting mixtures of 2-month-old Ly5^B6 and 2-month-old competitor Ly5^SJL BM. Thymocyte and granulocyte populations were analyzed for chimerism at least 10 weeks later. Bars represent percentages of Ly5^B6 cells within each population. Absolute numbers of thymocytes produced by Ly5^B6 BM were also similar (young: 77.1 × 10⁶ ± 19.3 × 10⁶; aged: 64.7 × 10⁶ ± 12.8 × 10⁶; P = .3). Data represent 11 chimeras in which 2-month-old BM was mixed with 2-month-old BM, and 22 chimeras in which 2-month-old BM was mixed with 18- to 24-month-old BM. Differences between young and old donors in the DP compartment were not significant. Error bars represent 1 SEM. (B) T-depleted BM cells (20 × 10⁶) from 2-month-old or 18- to 24-month-old Ly5^B6 donors were injected into nonirradiated 5-week-old Ly5^B6xLy5^SJL recipient mice. Mice were analyzed 13 to 16 weeks later for donor-derived chimerism in the DP thymocyte and blood granulocyte compartments. (C) Summary of data shown in panel B, representing 4 to 6 mice per group. Error bars represent 1 SEM (*P < .01). Absolute numbers of donor-derived thymocytes were also significantly different (young: 1.58 × 10⁶ ± 0.17 × 10⁶; aged: 0.32 × 10⁶ ± 0.054 × 10⁶; P < .001).