The quantitative relationship of the frequency of cells with radiation-induced chromosome aberrations in peripheral leukocytes in atomic bomb survivors has been evaluated as a function of gamma and neutron doses. Three different models have been examined; each assumes a linear or a non-linear response to gamma rays and a linear-response to neutrons. From the standpoint of the goodness of fit of these models, the model which “best” fits the data on the frequency of cells with radiation-induced chromosome aberrations is the exponit model, where the frequency of aberrant cells increases exponentially with dose. The goodness of fit for this model shows the frequencies of cells with any chromosome aberration or an exchange aberration to be dependent cubically on the gamma ray dose and linearly on the neutron dose. The result derived from the frequency of aberrant cells gives a conservative dose-response relationship. The relative biological effectiveness (RBE) of neutrons for frequency of cells with any chromosome aberration is calculated to be 13 at 30 rad of neutrons, 6 at 100 rad, 3 at 300 rad and so on.