HSCs are superior to LMPPs in reconstituting unconditioned X-SCID recipients with self-renewing stem and progenitor cells. BM cells were collected 16 weeks after primary transplantation from (n = 11) unconditioned X-SCID fetal recipients transplanted with 1000 CD45.1⁺ LMPPs () or 200 HSCs (). For each primary (first-degree) recipient, 1 femur equivalent was transplanted into 1 or 2 secondary (second-degree) sublethally (650 cGy) irradiated (8-10 weeks old) X-SCID (CD45.2) recipients (n = 18). Reconstitution analysis was performed at 12 weeks after second-degree transplantation. (A-C) Donor-derived contribution in first-degree and second-degree X-SCID recipients toward (A) total PB cells, (B) B lymphocytes, and (C) T lymphocytes, all presented as mean (SEM) percentages of total PB cells from 3 independent experiments. *P < .05.