To investigate the clinical applicability of prophylaxis of post-transplant graft-versus-host disease by UV-B irradiation of stem cell preparations, the UV-B sensitivities of human lymphocytes and primitive hematopoietic progenitors were compared. The mononuclear cell fractions (MNC) derived from human cord blood and granulocyte-colony-stimulating factor-mobilized peripheral blood were used. After UV-B irradiation, lymphocyte proliferation ability, hematopoietic colony-forming cells, and apoptotic cells were analyzed. At a dose of 33 J/m(2), significant differences were observed in the residual percentages of hematopoietic progenitors and lymphocyte functions [ANOVA, F (5,46) = 19.4; P <.0001], and the difference between CFU-C (85.2% + 24.0%; n = 8) and MLR (12.7% + 12. 6%; n = 10) was significant (P <.0001). There were no significant differences in the residual percentages of CFU-C, HPP-CFC, and LTC-IC. Percentages of annexin V-positive cells in the total MNC and the CD34(+) cell population in MNC after UV-B irradiation were 69.8% and 18.7%, respectively. In conclusion, there was a range of UV-B doses over which T lymphocytes were inactivated but hematopoietic progenitors, including HPP-CFC and LTC-IC, were preserved.