[en] In an attempt to induce in vitro transformation by radiation, several cell lines and primary cultures derived from embryonal tissues of hamster, mouse and man were tested. Under various conditions favorable for transformation, none of these were successfully transformed except for C3H mouse embryo-derived 10Tl/2 cells. Normally the cells contact-inhibited were irradiated with single graded doses and dispersed 3 hours after, followed by inoculation and 8-week cultivation with repeated medium renewals. A few types of focus were identified according to the description of Reznikoff et al. The foci characterized by (i) high cell density, (ii) increased affinity to a basic dye, and (iii) piled-up structure, were taken as an indication of transformation. The frequency of transformation was 6.5 x 10^-4 for 300 R which was 4 times higher than the frequency found in the untreated control. It increased dose-dependently until 500 R and then levelled off. Another type of experiment using TR cells derived from a leukemia-prone trisomy 21 human embryo, revealed that a single 300 R exposure to x-ray induced clones showing higher plating efficiency and plateau density than unirradiated control after 200 days of post-irradiation cultivation. However, the clones isolated did not show any particular transformational properties in vitro and tumorigenic activity on inoculation into nude mice. (author)