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The recovery of substrains with stable chromosome aberrations from irradiated fibroblast cultures are reported. Four human fetal cell strains were exposed to 600 rad of gamma rays at 200 rad/min. The efficiency of recovering viable cloned subpopulatiohs was approximately 87 %, and the frequency of clones with abnormal chromosomes was 40/100 colonies. G-band chromosome analyses for 34 abnormal substrains are described. Karyotypes of some of the clones with complex rearrangements are also presented. Analyses of a total of 47 aberrant events in the 34 abnormal substrains revealed a 7:1 and a 9:1 translocation-inversion and translocation-deletion ratios, respectively. Five of the abnormal sub-strains were continuously cultured; all except one showed signs of sensecence toward the end of 44 ± 10 doublings. Unusual prolonged proliferation capacity was observed in sub-strain FFS-1–9. The significance of this finding is discussed.