Radiation-induced effects were studied in leukocytes from 300 males 5-6 years after the Chernobyl accident. Radiation dose received ranged from 5 to 100 sGy. Leukocyte composition of blood was determined in blood smears stained according to Pappenheim and at flow cytometry on Facs tar laser cell sorter. Compared to controls, the majority of the patients had moderate relative and absolute lymphocytosis, eosinophilia and neutrophilopenia. Nuclear structure was changed more frequently in lymphocytes and neutrophils from the exposed subjects, especially in those who had received large dose. The findings evidence that 5-6 years after radiation accident the exposed subjects develop quantitative and qualitative shifts in leukocytes suggesting probable radiation effects on genetic system of the baseline hemopoietic cells.