The sensitivity of human natural killer cell activity after exposure of peripheral blood mononuclear cells to different doses of gamma irradiation was examined in a group of healthy adults and several families. Three patterns of radiation sensitivity were observed: 1) loss of all NK activity after 3000 rads irradiation; 2) loss of approximately 50% of the NK activity; and 3) maintenance of activity after this dose of irradiation. Low dose irradiation (500 to 2000 rads) resulted in an

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... of activity. The radiation dose giving low dose activation reflected the individual's sensitivity to 3000 rads. Population studies and the segregation in two informative families indicate that radiation sensitivity of NK activity is controlled by X-linked codominant gene.