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Bone-marrow cell chromosome aberrations were studied in 14 unirradiated control patients with malignant tumors and 132 comparable patients after a single localised exposure to 60Co γ irradiation. In control patients the chromosome break frequency was 0.01 per cell and aberrant cell frequency was 1.1%. In 79 patients the dependence of chromosome aberrations on the time interval after irradiation following a dose of 250 rad was studied. The highest frequency of aberrant cells was observed 18 and 21 h after a single irradiation (about 65%). It was shown that the number of chromosome aberrations in cells entering mitosis soon after irradiation (apparently the G2 cells) was much greater (438.1 aberrations per 100 cells) than that in cells entering mitosis in other periods. A sharp decrease in the number of aberrant cells and aberrations was observed 72 and 96 h after irradiation, and especially at 120 and 168 h. In 67 patients the relation between dose and chromosome aberration yield was studied. It was shown that the yield of chromosomally aberrant cells increases linearly with dose, according to the equation Y = 6.385+0.195 s D .