Cultured human lymphocytes exposed to very low doses of X-rays become less susceptible to subsequent higher doses of X-rays. Cells exposed to doses as low as 0.5 rad (cGy) or 1 rad of X-rays at 32-34 h of culture become adapted so that less cytogenetic damage in the form of chromosome breakage is induced by 150 rad administered at 48 h. This response, which does not occur after high initial doses of X-rays, can be eliminated by 3-aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase.