CD25⁺ and CD25^-CD4⁺ T cells support donor splenic CFU-GMs 7 days after BMT, but CD4⁺CD25⁺ T cells do not lead to GVHD. B6-GFP BM-TCD (2 × 10⁶, A; 1 × 10⁶, B) was transplanted alone or with 1 × 10⁶ CD25⁺ or CD25^-B6 CD4⁺ T cells into 7.0-Gy irradiated BALB/c recipients. Recipients were killed 7 days after BMT for total splenic donor CFU-GM analysis. Data presented are mean CFU/spleen ± SEM of pooled splenocytes and 3 independent experiments (A). Error bars in B represent mean and SEM of individual mouse weights. Groups of BM transplant recipients (n = 6-7/group) were monitored to assess the development of GVHD-associated weight loss (B) and mortality (C). □ indicates BM only; ♦, BM + CD4⁺CD25⁺; ▿, BM + CD4⁺CD25^-. ^*** indicates significance of P < .001 compared with BM transplanted alone.