This study was undertaken to investigate the biological effect of monoenerg
etic neutrons on human lymphocyte DNA and chromosomes. Monoenergetic neutro
ns of 2.3, 1.0, 0.79, 0.57, 0.37 and 0.186 MeV were generated, and Cf-252 n
eutrons and Co-60 gamma-rays were also used for comparison. Biological effe
ct was evaluated two ways. The RBE values with the comet assay were estimat
ed as 6.3 and 5.4 at 0.37 MeV and 0.57 MeV relative to that of 60Co gamma-r
ays, and chromosome aberration rates were also observed in these different
levels of monoenergetic neutrons. The yield of chromosome aberrations per u
nit dose was high at lower neutron energies with a gradual decline with 0.1
86 MeV neutron energy. The RBE was increased to 10.7 at 0.57 MeV from 3.9 a
t Cf-252 neutrons and reached 16.4 as the highest RBE at 0.37 MeV, but the
value decreased to 11.2 at 0.186 MeV. The response patterns of initial DNA
damage and chromosome exchange were quite similar to that of LET. These res
ults show that the intensity of DNA damage and chromosomal exchange is LET
dependent. RBE of low energy neutrons is higher than that of fission neutro
ns. Low energy neutrons containing Hiroshima atomic bomb radiation may have
created a significantly higher incidence of biological effect in atomic bo
mb survivors.