To characterize the differences in the radiosensitivity of individual populations of human hematopoietic stem/progenitor cells (HSPCs), we examined the relationship among cell surface antigens, clonogenic potential and radiation survival. The expressions of CD34, CD38, CD45RA, CD110 and Tie-2, early differentiation pathway-related antigens in hematopoiesis, were analyzed on the surface of HSPCs enriched for CD34 antigen expression in 20 samples prepared from human placental/umbilical cord blood. A significantly positive relationship was observed between CD38 antigen and CD45RA and between CD110 and Tie-2. No significant relationship was observed in most cases among the antigens and the number of colony-forming cells (CFCs); however, the number of megakaryocytic progenitor cells correlated negatively with the percentage of Tie-2 cells. The percentage Tie-2 cells correlated significantly with the surviving fraction of CFCs irradiated with 2 Gy of X rays, suggesting that the radiosensitivity of individual CFC populations is related to the percentage of Tie-2-expressing cells. In addition, the number of progenitor cells closely correlated with the surviving fraction after 2 Gy of X rays. These results suggest that the radiosensitivity of individual HSPC populations is related to the number of progenitor cells in the population especially dependent on the presence of immature HSPCs such as Tie-2 cells.