[en] The effect of X-rays and different-energy neutrons on human bone-marrow cells was studied using two different cell-culture techniques - diffusion chamber (DC) growth and colony formation in vitro (CFU-C). Based on the survival and proliferative granulocytes in DC on day 13, the D₀ value was 80 rad with X-rays, and 117 rad as measured by the CFU-C assay. The D₀ values for neutrons depended on the radiation source and the energy level. The r.b.e. values, which dropped with increasing energy levels of mono-energetic neutrons, were (i) 0.44 MeV; DC 3.7, CFU-C 4.1; (ii) 6 MeV; DC 1.8, CFU-C 2.0; (iii) 15 MeV; DC 1.6, CFU-C 1.6; (iv) fission neutrons; DC 2.6, CFU-C 2.4. (author)