The 50 per cent lethal flux of neutrons at energies up to 3 MeV is calculated from the assumptions that (a) the 50 per cent lethal dose of γ rays is 400 r, (b) the relative biological efficiency of protons is 6·5, and (c) that the criterion for lethal effects is the equivalent amount of ionization produced in the whole body. For isotropic incidence the 50 per cent lethal flux varies from 1·6 × 1012 n/cm2 at thermal energy to 1·7 × 1010 n/cm2 at 3 MeV.