MBEC transplantation provides radioprotection of BM progenitors and augments the regeneration of stem cell–enriched populations. BM mononuclear cells were collected from mice on days +10, +15, and +20 following 700 cGy of total body irradiation. The percentages of c-Kit⁺sca-1⁻lin⁻ progenitors and the more primitive KSL cells were measured by flow cytometry in the BM at each time point in normal mice (A), in 700 cGy-irradiated control mice, and in mice treated with MBECs by intravenous or via intraperitoneal administration for 5 days. As shown in panel B, control mice demonstrated marked reduction in viable c-kit⁺sca-1⁻lin⁻ progenitors at day +10, whereas all animals treated with MBECs maintained significantly higher numbers of this population following radiation exposure. At days +15 (C) and day +20 (D), both groups of MBEC-treated animals demonstrated recovery of KSL cells compared with untreated controls. Percentages shown in the upper quadrants of each figure represent the percentage of that population within the lin⁻ subset.