Recipients of syngeneic bone marrow and FoxP3⁺ T cells exhibit diminished CFU-IL3 progenitor cell activity after transplantation. CD4⁺GFP⁺ (Treg) cells were obtained from B6 Foxp3^gfp donors after positive selection via cell sorting for allophycocyanin (CD4) and gfp (FoxP3) expression; purity = 99.8% (A). These highly purified Tregs were coinfused with syngeneic B6 TCD-BMCs at day 0 into lethally irradiated (9.5 Gy) B6 mice (n = 4). Control recipients received TCD-BM alone (n = 4). One week after HSCT, CD4⁺ GFP⁺ Tregs were readily identified in recipient spleens (B). Representative dot plots of control and experimental recipients are shown. Values in dot plots represent percentage of gfp⁺ cells. Control, 0%; experimental, 2.4%-2.9%. CFU-IL3 activity by spleen cells was assessed at day 7 after HSCT from triplicate cultures as described (C). Data are presented as total numbers of CFU/spleen from B6 animals that underwent transplantation. Statistical analysis comparing the mean values of the 2 transplant groups: P < .02.