The sublethal radiation damage repair capacity of primary clonogenic blasts [i.e., leukemic progenitor cells (LPC)] from 74 newly diagnosed acute lymphoblastic leukemia (ALL) patients was analyzed using LPC colony assays. We determined the (a) Dq (quasithreshold dose) and n (extrapolation number) values from single dose radiation survival curves constructed according to the single-hit multitarget model of cell survival, (b) beta values and alpha:beta ratios from single dose radiation survival curves constructed according to the linear quadratic model of cell survival, and (c) recovery factor values from survival data in split-dose experiments. Clonogenic blasts from different ALL patients varied substantially in their ability to repair sublethal radiation damage. However, in 11 of 30 (37%) T-lineage ALL cases and 13 of 44 (30%) B-lineage ALL cases, the radiation survival curves of fresh LPC were characterized by a distinct initial shoulder, providing circumstantial evidence that LPC from a significant portion of ALL patients (24 of 74 cases = 32%) are able to repair sublethal radiation damage. Leukemic progenitor cells from 23 of 34 (68%) evaluable cases evaluated had alpha:beta ratios of < 5 Gy, indicating that they may possess a substantial capacity to repair sublethal radiation damage. In order to further elucidate the repair capacity of ALL LPC, we compared the antileukemic efficacy of fractionated irradiation with 2 x 2 Gy to the antileukemic efficacy of single-dose irradiation with 1 x 4 Gy. In 20 of 28 cases (71%), a > or = 20% increase in LPC survival (recovery factor > or = 1.2) was observed when 4 Gy were delivered in two doses instead of a single dose, providing direct evidence that ALL LPC are able to repair sublethal radiation damage. Our results indicate that modifications in total body irradiation regimens, currently administered prior to bone marrow transplantation, may be necessary to diminish the probability of relapse in high-risk ALL.