Transplantation of mESC-derived TEPs improves thymic reconstitution in aged allo-BM transplant recipients. Lethally irradiated 12- to 14-month-old or 1-month-old 129B6F2 mice were injected intrathymically with mESC-derived EpCAM1⁺, EpCAM1⁻ cells, or PBS, and intravenously with TCD BM from BALB/c mice. Thirty days after BMT, the numbers of (A) total thymocytes and their subsets, (B) donor-origin ETPs (lin⁻IL-7Rα ⁻c-Kit⁺CD44⁺CD25⁻), and (C) total TECs (CD45⁻EpCAM1⁺MHC II⁺) and their subsets cTECs (CD45⁻EpCAM1⁺MHC II⁺Ly51⁺) and mTECs (CD45⁻EpCAM1⁺MHC II⁺Ly51⁻) were quantified by flow cytometry and compared with those in non-BMT control or EpCAM1⁺ cell–treated BMT young mice. Means ± SDs are presented (n = 5-6). Mean percentages of donor-origin thymocytes in mESC-derived EpCAM1⁺ cell–, EpCAM1⁻ cell–, or PBS-treated BMT aged mice and EpCAM1⁺ cell–treated young mice are 86.9%, 87.2%, 87.7%, and 85.9%, respectively. **P < .01 and ***P < .001. The data are representative of 3 independent experiments.