A final series of experiments on tumor induction by repeating 90Sr-90Y beta irradiation of the back of ICR mice three times weekly throughout the life span showed no tumor incidence with repeated doses of 0.5 Gy per exposure. The whole dose-response relationship for repeated irradiation with 0.5 to 11.8 Gy per exposure, summarized along with data obtained previously, showed a unique nonlinear dose response with saturation of tumor incidence at doses of over 2.5 Gy per exposure and a threshold-like value around 1.5 Gy per exposure.