Human and hamster malanoma cells were irradiated is vitro with single and fractionated daces of γ-rays. Except for a tendency to high extrapolation numbers, survival curves did not show particular radioresistance. D2-D1 was between 220 and 300 rad. Regrowth delay after split dose irradiation to the hypoxic Hardiog-Passey malanoma in vivo yielded a D2-D, of 700 rad. While the tumor control dose for 50% (TCD-50) was within normal range (4,400 rad) Harding-Passey malanom as regressed very slowly after irradiation and often took months to clear away.