[en] Cells of fibroblastic origin were grown in Petri dishes and dishes were x-irradiated after plating cells at a density such that about 400 viable cells per dish survived the various treatments. Results of single dose experiments showed that survival, as a function of dose above 200 rads, decreased exponentially. The transformation frequency increased rapidly from 10 to 400 rads and then reached a plateau from 600 to 1000 rads. In a split dose experiment the transformation frequency increased rapidly from 300 to 400 rads and then decreased slowly