The effects of acute exposure to low- and high-dose radiation on the quantitative and functional parameters of the immune system were analyzed. C57BL/6 mice were irradiated with different doses of γ radiation (0.01, 0.05, 0.1, 0.5 and 2 Gy) and splenocytes were isolated at various times. Alterations in the distribution and surviving fraction of splenocyte subsets such as CD4(+) and CD8(+) T lymphocytes, regulatory T cells (Treg), natural killer (NK) cells, dendritic cells (DCs) and B lymphocytes were analyzed by flow cytometry. Apoptosis frequency was quantified by the TdT-mediated dUTP-biotin nick end labeling (TUNEL) method 4 h after irradiation. Cytokine expression was investigated by real-time reverse transcription-polymerase chain reaction (RT-PCR). Low doses decreased apoptosis in the splenocyte subpopulations studied most prominently in NK cells and DCs. Exposure to 2 Gy increased apoptosis in all splenocyte subpopulations; B cells were the most sensitive and NK cells and DCs the least sensitive. The lowest cell numbers were measured 3 days after irradiation, with minor changes by day 7. CD8(+) and B cells were rather resistant to low doses but were very sensitive to 2 Gy, while NK cells, DCs and Treg cells were much more resistant to high doses. Expression of the T-helper 1 (Th1)- and helper 2 (Th2)-type cytokines decreased after low doses and increased after high doses. Interleukin 6 (IL-6) reacted at early times and IL-10 at later times. IL-5 levels were consistently elevated. These data highlight the differences in the responses of different splenocyte subpopulations to low- and high-dose radiation.