The effects caused by low-dose irradiation differ from those caused by high-dose irradiation. At present only little is known about the long-term effects of chronical exposure of individuals and whole populations to low-dose irradiation. This problem particularly concerns people living in territories heavily contaminated with radioactive material due to Chernobyl Nuclear Power Plant accident. We investigated different parameters of the peripheral blood from 124 men, working at the "Shelter" object (demolished block IV of the nuclear power plant). In many cases the accumulated radiation dose (gamma-rays) was more than 50 rem. As control group we examinated 302 male donors (of the same age) from the Kiev blood-transfusion station. The studies revealed the following effects in the "Shelter" object workers: an increased manifestation of signs of common-inflammatory reactions, such as an increased number of leukocytes and absolute number of white blood cells, an unproportionally high number of monocytes and band neutrophiles, and an increase in plasma cytokine levels (interferon-alpha, tumor necrosis factor-beta but not interleukin-1beta). Ongoing studies are targeted to develop and introduce new methods and techniques to measure the frequency of DNA strand breaks in peripheral blood lymphocytes for the surveillance of persons working and/or living in a radioactively contaminated environment after Chemobyl accident.