[en] Autochthonous tumors produced by subcutaneous injection of 3-methylcholanthrene in mice were subjected to treatment by single X-irradiation. Tumors, fibrosarcomas, showed recurrence after temporary regression even with doses up to 7.5 Krad. To the contrary, tumor transplants from the same source were cured by irradiation [TCD₅₀ (120 days) = 4.4 Kard]. Spontaneous mouse mammary tumors reported in literatures are also curable with a similar TCD value. The critical problem in treatment of chemically induced tumors whether the observed recurrence is due to true recurrence of radioresistent tumor cells or to induction of a new second tumor was discussed. (author)