Split-dose irradiation of mice with two doses of 0.5 Gy given at 2 to 5 h interval damages haemopoietic stem cells (CFU-S) less than single exposure to 1 Gy. Also in comparison with whole-body single exposure of mice to a high sublethal dose of 7 Gy, splitting this dose into two doses of 3.5 Gy given at 4-h interval brought about more rapid recovery of CFU-S in femoral bone marrow and of bone marrow cellularity. At 360 days after irradiation with 7 Gy gamma-rays, a secondary decrease in CFU-S was apparent in bone marrow, which was more profound in the single-exposure group than in the group given split doses amounting to 7 Gy. The protective effect of short-term dose splitting was also evident in an investigation of mortality of irradiated mice. The LD50/30 value in mice exposed to a single dose of gamma-rays (8.3 Gy) increased to 10.5 Gy, when the dose was split (two doses, 4-h interval).