The dose-response relationship of chromosome aberrations in the peripheral blood lymphocytes was studied in blood samples obtained from the patients who were exposed to a single dose of 60Co γ-rays for their malignant diseases. The level of chromosome aberration yield, as studied 24 hours after irradiation, was not simply related to the radiation dose per se, but body weight, irradiated volume and radiation-induced inactivation of lymphocytes were found to be other important factors intimately related to the frequencies of chromosome aberrations; aberration yield expressed on the simple per-cell basis was roughly proportional to the irradiated volume and inversely proportional to the body weight. The intercomparison of the frequency of chromosome aberrations in these locally irradiated persons with that obtained by the in vitro experiments suggested the D37 value of about 350 rads for the survival of lymphocytes as measured 24 hours after irradiation.