Chromosome translocations were analyzed by chromosome painting techniques using No. 12 and 4 whole chromosome painting probes to examine the effect of high-level natural radiation on the frequencies of chromosome translocations in human peripheral lymphocytes. The subjects were 5 children and 13 old men from the high background radiation area and 6 children and11old men from the control area. Children had lesser translocations and smaller variation compared with the old men. The frequency of translocations among the old subjects varied widely from person to person. No difference of the frequency of chromosome translocations between high background radiation area and control area. No effect of high-level natural background radiation on the frequencies of stable chromosome translocation was detected.