In the Chinese hamster cell line CHEF-125, sister chromatid exchanges occurred at a rate of a little higher than one per three chromosomes for each cell cycle. The exchanges were detectable by labeling with H3-thymidine and autoradiographic analyses of chromosomes at the second and subsequent metaphases after labeling had occurred. To test the hypothesis that sister chromatid exchanges are caused by radiation, cells were incubated in media with different amounts of H3-thymidine. No statistically significant change in the exchange rate was detected over 100-fold range of variation in the amount of incorporated H3-thymidine (determined by grain counts of autoradiographs). We have concluded that sister chromatid exchanges are not caused by tritium radiation and therefore are spontaneous events. Cultures were also irradiated with acute doses of x-rays up to 200 r and scored for sister chromatid exchanges. Between zero and 50 r there was a statistically significant increase in the rate of exchanges. This is interpreted as evidence that x-rays can induce some exchanges, although the majority of these events are probably spontaneous.

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