The radiosensitivity of E. coli AB2463 recA, given as the reciprocal of the mean lethal dose, Do-1, has been shown to be the same for four fast neutron beams with widely different energy spectra. It is proposed that this organism can be used to intercompare dosimetry on fast neutron beams with mean energies in the range 4 to 25 MeV with an accuracy of +/- 5%.