Fibroblasts were established in vitro from skin biopsies obtained from 55 women and 1 man with or without breast cancer and with or without exposure to radiations from the atomic bomb (A-bomb) explosion in Hiroshima. The radiosensitivity of these cells was evaluated by clonogenic assays after exposure to X-rays. Dose-response curves were fitted to a multitarget model, S/S0 = A[1-(1-e-D/D0)N]. There was no difference in the means or variances of radiosensitivity between exposed and nonexposed groups, or between groups with and without breast cancer.