The aim of this study was to evaluate whether or not low radiation doses can induce an adaptive response in human lymphocytes exposed in vivo. We examined chromosome aberrations, micronuclei, radiosensitivity, and apoptosis in the peripheral blood of young radiation workers (mean age 35.8 +/- 9) with 14.9 +/- 5.2 years of occupational exposure to ionizing radiation. Adaptive response was estimated using two parameters of radiosensitivity: micronuclei and apoptosis. Baseline and radiation-induced micronuclei (2 Gy, 60Co gamma-rays) were assessed with the cytochalasin block micronucleus (CBMN) test. Cytotoxicity was analyzed by flow cytometry for human white blood cells identifying cells that displayed apoptosis-associated DNA condensation. In the exposed group, eight out of 34 cases were dicentric-positive. A statistically significant difference (t = 3.1, p less 0.01) was found between the dicentric-positive and dicentric-negative group in the yield of baseline micronuclei. However, the yield of radiation-induced micronuclei between these groups did not differ significantly (t = -1.57, p less 0.15). An inverse correlation between baseline micronuclei and apoptosis group was noted in the dicentric-positive group (r = -0.53, p less 0.45). Our study showed that occupational exposure to low doses of ionizing radiation failed to induce adaptation of blood lymphocytes to the challenge dose. An inverse relationship between baseline micronuclei and apoptosis in the dicentric-positive group indicates that chronic exposure to low doses of ionizing radiation depresses the ability of leukocytes to undergo apoptosis. Our study also showed that the effects of radiation-induced cytotoxicity in occupationally exposed persons could be determined by flow cytometry.