When eliminating the consequences of large-scale radiation accidents, primary triage of victims is of key importance during the early phase of medical evacuation. Information about lymphocyte counts (blood test) per unit of peripheral blood volume can be used for this purpose. The study was aimed to validate the method of using a lymphocyte test for prediction of acute radiation injury severity in the first days after the exposure associated with the radiation mass casualty incident, given peripheral blood was tested once. We performed correlation analysis of the data of laboratory studies focused on quantifying lymphocytes in peripheral blood of victims during the first days following the Chernobyl disaster and other radiation accidents on the territory of the countries of the former USSR (115 individuals), including radiation accidents with gamma neutron radiation (20 individuals). It was found that with the lymphocyte concentration of 0.2–1.0 × 10⁹/L on day 2 after exposure, the absolute error of estimated dose was ±1.5 Gy in case of gamma exposure and ±1.3 Gy in case of exposure to gamma neutron radiation. When the lymphocyte concentration exceeds 1.0 × 10⁹/L, mild acute radiation syndrome (ARS) is predicted, given the average dose is below 2.0 Gy; when the lymphocyte concentration is less than 0.2 × 10⁹/L< the estimated average dose exceeds 4.0 Gy, which corresponds to severe or extremely severe ARS. Thanks to the lymphocyte test accessibility and simplicity, this biological dosimetry method can occupy a worthy position in the diagnosis of radiation injury associated with large-scale accidents, since the results of cytogenetic tests are not available within first days after the accident.