In vivo conversion of CD4⁺ foxp3⁻ T cells to CD4⁺ foxp3⁺ T cells is negligible during acute GVHD. Lethally irradiated bm12 (1000 cGy; n = 8-11/group) or B6 (1000 cGy; n = 8-12/group) mice received a transplant of B6 Rag-1 BM (5 × 10⁶) and sorted CD4⁺ EGFP-foxp3⁻ T cells (0.6 × 10⁶). Cohorts of mice were killed on either days 7 to 9 or day 20 after transplantation. Spleen cellularity (A,E), absolute number of splenic CD4⁺ T cells (B,F), percentage of CD4⁺ EGFP⁺ T cells in the spleen (C,G), and absolute number of CD4⁺ EGFP⁺ T cells in the spleen (D,H) are depicted. Data are presented as the mean (± SEM) and are the cumulative results from 2 to 3 independent experiments. (Statistics: **P < .01.)