The suitability of comet assay to identify DNA damage induced by neutrons of 
varying energy was tested. For this purpose, monoenergetic neutrons from 
Hiroshima University Radiobiological Research Accelerator (HIRRAC) were used to 
induce DNA damage in irradiated human peripheral blood lymphocytes. The level of 
damage was computed as tail moment for different doses (0.125-1 Gy) and compared 
with the effects resulting from irradiation with (60)Co gamma. The 
neutron-irradiated cells exhibited longer comet tails consisting of tiny pieces 
of broken DNA in contrast to the streaking tails generated by (60)Co gamma. The 
peak biological effectiveness occurred at 0.37 and 0.57 MeV; a further increase 
or decrease in neutron energy led to a reduced RBE value. The RBE values, as 
measured by the comet assay, were 6.3, 5.4, 4.7, 4.3, 2.6, and 1.7 for 0.37, 
0.57, 0.79, 0.186, 1, and 2.3 MeV neutrons. The lower RBE value obtained by the 
comet assay when compared to that for other biological end points is discussed. 
This study reports the usefulness of the alkaline comet assay for identifying 
DNA damage induced by neutrons of the same radiation weighting factor. The comet 
assay is a potential tool for use in neutron therapy, as well as a method for 
the rapid screening of samples from individuals accidentally exposed to 
radiation.