[en] The purpose of the study is to determine the extent of the genetically toxic effect caused by ionizing radiation, assessed with the micro-nuclear method in lymphocytes from the peripheral blood of the long term workers at the Kozloduy NPP. 17 persons age 32-55 with length of service from 8 to 25 years, received summary dose from 254.17 to 423.1 mSv, have been studied. Data for the frequency of the occurrence of micro-nuclei in the lymphocytes in the peripheral blood of the studied persons is presented. The genetically toxic effect is found. From the obtained results the following conclusions are made: the sensibility of the micro-nuclear test as a biological indicator of risk groups submitted to ionizing radiation is verified; potential risk of development of late consequences for the studied persons exists; non specific prophylactics and specialized observation until the end of service and also at pension age are recommended