T he radiation sensitivity of primary clonogenic blasts from 44 children with newly diagnosed B-cell precursor acute lymphoblastic leukemia (ALL) was analyzed using leukemic progenitor cell (LPC) colony assays. The derived values for SF2 (surviving fraction at 200 cGy) and alpha (initial slope of radiation survival curves constructed according to the linear quadratic model) indicated a marked interpatient heterogeneity in intrinsic radiation sensitivity of LPC populations. The SF2 values ranged from 0.01 to 1.00 (median = 0.430; mean +/- SE = 0.47 +/- 0.04), and the alpha values ranged from 0.000 to 3.272 Gy-1 (median = 0.280 Gy-1; mean +/- SE = 0.430 +/- 0.093 Gy-1). When CD19+ CD34+ versus CD19+ CD34- immunophenotypes were compared, a trend toward higher SF2 and lower alpha values were observed in LPC from CD34+ patients, consistent with greater radiation resistance. When patients were divided into three approximately equal groups based on increasing levels of CD34 expression, a clear ordering effect was observed indicating that increased CD34 expression levels are associated with significantly higher radiation resistance at the level of B-lineage LPC. The highest CD34 expression group (> or = 75% positivity) had 1.4-fold higher SF2 (P = 0.05) and twofold lower alpha values (P = 0.06) than the lowest group (< 30% positivity). Furthermore, the CD34 positivity of radiation resistant (alpha < or = 0.2 and SF2 > or = 0.5) B-cell precursor ALL cases was greater than the CD34 positivity of radiation sensitive (alpha > 0.2 and/or SF2 < 0.5) cases (56 +/- 9% versus 34 +/- 9%, P = 0.09). Whereas only 6 of 16 (38%) of radiation sensitive cases were CD34+, 11 of 15 (73%) of radiation resistant cases expressed CD34 (P = 0.04). Our results offer new insights into the inherent and/or acquired radiation resistance of primary clonogenic blasts from B-cell precursor ALL patients.