The average number of chromosome aberrations per cell decreases by 40 and 50%, respectively, when human peripheral blood lymphocytes are cultured for 3 and 5 days after exposure to 400 rads of gamma rays and before stimulation with phytohemagglutinin (PHA). The decrease is due primarily to a loss of cells with multiple aberrations. The results are consistent with the hypothesis that a chromosomal radiosensitive subpopulation of PHA-responsive cells is eliminated preferentially by radiation. Examination of the survival of T-cells, which are PHA-responding cells, 4 days after exposure to graded doses of gamma rays suggests the presence of two subpopulations differing in radiosensitivity, in support of the hypothesis. Also in support are the results of chronic radiation studies which show that the yield of aberrations is independent of dose rate over the range of 11-59 rads/hr, if the dose is delivered in 24 hr or less, a time when cell loss is small. The decrease in aberration yield that has been observed when the irradiation is extended over 3 days is probably then the result of selection against radiosensitive cells.

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