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Golden hamster embryo cells were used to study morphological transformation induced by low doses of X rays at different dose rates. X irradiation at lower dose rates was less effective in cell killing and induction of transformation than at higher dose rates. The general shapes of induction curves of transformants were almost the same at all dose rates. At shoulder doses (1 to 150 R), the transformation frequencies increased steeply with increasing dose at all dose rates. At doses higher than 150 R survivals declined exponentially, but the frequencies of transformants increased only slightly. However, irradiation at higher dose rates was more effective in induction of transformants than that at lower dose rates. We conclude that transformational damage introduced at a low dose rate, as well as sublethal damage, may be repaired during low-dose-rate irradiation, but transformational damage may be different from sublethal damage.

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