Nuclear anomalies of different types appear in cells in response to the action of ionizing radiation after the passage of the first mitotic division. In this article, we present the results of the study of the frequency of occurrence of three types of nuclear anomalies ("tailed" nuclei, nucleoplasmic bridges, and dumbbell-shaped nuclei) in vitro in human lymphocytes cultured with cytochalasin B when exposed to X-rays at doses of 0.0, 0.1, 0.2, 0.4, 0.5, 0.7, 1.0, 1.5, and 2.0 Gy. To stop the cell cycle of cultured lymphocytes after the first mitotic division, a cytokinesis block was performed using cytochalasin B. Dose-dependent curves of the occurrence of lymphocytes containing "tailed" nuclei, nucleoplasmic bridges, or dumbbell-shaped nuclei after irradiation have been constructed. At the same time, frequencies of occurrence of chromosomal aberrations (dicentric and ring chromosomes) in the culture of lymphocytes exposed to the same radiation doses were studied. Comparison of the frequencies of occurrence of dicentric and ring chromosomes with frequencies of occurrence of nuclear anomalies allows us to conclude that these nuclear anomalies are formed as a result of chromosomal aberrations arising in lymphocytes under the action of ionizing radiation. More than that, most of the chromosomal aberrations are converted into dumbbell-shaped nuclei in vitro in the culture of lymphocytes in the cytochalasin block.