Recipients of syngeneic bone marrow and Tregs have decreased CFU-IL3 activity in the marrow compartment 2 weeks after HSCT. Enriched CD4⁺CD25⁺ B6 Treg preparations were isolated from IL-2/anti-IL2 mAb complex–treated animals and coinfused with syngeneic TCD-BMCs on day 0 into lethally irradiated (9.5 Gy) B6 mice. (A) Approximately 2 weeks (days 14 and 17) after transplantation, markedly enhanced levels of CD4⁺CD25⁺ T cells (∼4×) were identified in the marrow compartment from recipients receiving Tregs and BMCs (experimental) versus BMCs (control) alone. The majority of these CD4⁺CD25⁺ T cells were FoxP3⁺ (inset histograms: isotype-FITC Ig staining; dotted line represents gate based on unstained control sample). (B) CFU-IL3 numbers were assessed from triplicate cultures as described. Results are presented as the mean CFU-IL3 numbers/marrow compartment in control (n = 6) and experimental (n = 6) groups. Error bars represent average CFU-IL3 ± SD from 18 replicate cultures. P < .05.