Frequency of the appearance of binuclear cells with nuclei having outgrowth into the cytoplasmic space and arisen after first mitosis in human lymphocyte culture is linear-square dependent on the X-irradiation at doses from 0.0 to 4.0 Gy. Positive correlation between frequency of cells with 'tailed' nuclei and frequency of metaphases of first mitosis having dicentrics and rings was established. Apparently, formation such 'tailed' nuclei is connected with dicentrics and rings.