Regeneration of progenitor cells in bone marrow (upper panel) and spleen (lower panel) at 7, 10, 13, and 17 days after the last fraction of TBI in mice irradiated with three fractions of 3 Gy with 24 hour intervals. (▪), GM-CFU per femur or spleen; (▧), BFU-E; (□), CFU-Megs per femur or spleen. Placebo, control mice; TPO-2h, mice treated with 0.9 μg TPO 2 hours before the first fraction of TBI; TPO+ 2h, mice treated with 0.9 μg 2 hours after the last fraction of TBI; TPO3x+2h, mice treated with three doses of 0.3 μg/mouse 2 hours after each fraction of TBI, three mice per data point. SDs were calculated for the sum of the individual colonies per mouse. *Significantly different from time matched controls (P < .05), **(P < .005).