[en] Bone marrow transplantation has often been closely linked with accidental or intentional therapeutical irradiation. In both situations, study of the radiosensitivity of human blood cell subsets is of interest. Using one-color flow cytometry analysis of B lymphocytes, T cell subsets, and natural killer cells, we previously reported that lymphocyte subsets exhibit equal radiosensitivity. Taking advantage of recent developments in the knowledge of leukocyte differentiation antigens and flow cytometry technology we undertook a study of blood cell subsets to search for rare populations exhibiting different radiosensitivity. Thirty patients, who were delivered a 12 Gy fractionated total body irradiation as part of their conditioning regimen before transplantation for malignant disorders, were studied using multicolor flow cytometry. T and B lymphocytes showed a sharp, radiation-induced decrease, with the B lymphocytes (cluster of differentiation (CD) 19+) being the most sensitive. When analyzed by multicolor flow cytometry all major lymphocyte subsets appeared equally sensitive to the in vivo irradiation. Therefore, all major lymphocyte subsets sharing the helper phenotype (naive or memory) and the cytotoxic phenotype appeared equally sensitive to in vivo whole body irradiation. In parallel, the CD34+ cell subset remained basically unchanged after whole body irradiation. Finally, the CD3-, 56+, 16+ natural killer cell subset was relatively radioresistant (91 and 74% of its initial value, after 2 and 4 Gy, respectively) as compared to other lymphocyte subsets. Our study provides evidence that T and B cell subsets seem to be highly radiosensitive in vivo. The CD34+ progenitor/stem cells and NK cells seem to be more radioresistant. This latter result might provide clues to the understanding of the pathophysiogeny of radiation-induced aplasia and of the engrafment/rejection process following bone marrow transplantation. 20 refs., 3 figs., 1 tab