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Introduction. In the event of mass radiation incidents, medical triage of exposed individuals must enable rapid identification of individuals at risk of developing the hematopoietic form of acute radiation sickness (ARS). Objective. Development of effective cytogenetic criteria for identifying individuals at increased risk of developing the hematopoietic form of ARS during mass radiation incidents. Materials and methods. The study involved 12 donors aged 23–73 years. The study object was phytohemagglutinin-stimulated T-lymphocytes from peripheral blood. Blood samples were exposed to in vitro gamma-irradiation at doses of 1 Gy and 2 Gy using an IGUR-1M unit. Cytogenetic preparations were obtained according to a standardized cytogenetic protocol and stained with a 2% Giemsa solution. Images were digitized and analyzed using the Metafer/Ikaros (Metasystems, Germany) software package. The frequency of chromosomal aberrations per cell was assessed. Metaphases of T-lymphocytes with dicentric chromosomes were counted in the order of their occurrence during slide analysis. For statistical analysis, Past 4.01 and SPSS Statistics 21 software packages were used. Results. The decision regarding the classification of a potentially irradiated sample as one falling into the dose range likely to cause ARS development should be based on an analysis of the number of T-lymphocyte metaphases required to identify five dicentric chromosomes. In the study, dicentric chromosomes were detected in samples without irradiation in 33% of the examined individuals. After in vitro irradiation of blood samples at a dose of 1 Gy, the average frequency of dicentric chromosomes was 0.073 ± 0.008 per cell, reaching 0.28 ± 0.02 per cell at a dose of 2 Gy. Conclusions. A preliminary algorithm for differentiating in vitro irradiated cytogenetic samples into dose ranges has been developed. The identification of the fifth dicentric chromosome in order of analysis within the first 26 T-lymphocyte metaphases served as the basis for assigning the sample to a dose range of 2 Gy and above. In cases where the fifth dicentric chromosome was identified between the 27 th and 85 th metaphase, the sample was assigned to a dose range of 1 Gy and above. In cases where fewer than five dicentric chromosomes were detected upon analysis of 85 metaphases, the sample was assigned to a dose range below 1 Gy. Future research will be aimed at refining the algorithm and validating the results.