It is well known that fluorescence in situ hybridization (FISH) is a powerful method largely used for detecting many chromosomal rearrangements and translocations, which are important biomarkers for the dose assessment, particularly in cases of the not well defined time of the subject’s exposure to ionizing radiation. In 2008 one person suspected himself of an accidental exposure to ionizing radiation, but the employer was informed about this accident in 2009. Here we present an example of the work, the aim of that was to estimate the absorbed dose of this presumably exposed to radiation person, in cells obtained from the subject about a one year after the accident. The aim for RADIPER participant’s training was enabling them to learn the results of this practical application of the translocation analysis for the purpose of the retrospective biological dosimetry. Then, during the training participants were to estimate the potential dose of the exposed anonymous subject, on the base of the analysis of cellular responses given in two sets of images of the aberrations in chromosome pairs: 1, 2 and 4 (the first one set was simulating results of the overexposure, and the second one set simulating results equal to the average level of the frequencies evaluated in our research for the control group of unexposed persons), and to compare its own results of the observed and detected translocations frequency in chromosome 1 with the investigated earlier response relationship. The chromosome pairs 1, 2 and 4 were treated using the three-color FISH technique. Stable chromosomal aberration, especially translocations, were scored, in the 60 images for each investigated case, by 10 course participants and the absorbed dose was reconstructed on the basis of a calibration curve established earlier in our laboratory for lymphocytes irradiated in vitro. The simulated absorbed dose was estimated by participants below 2 Gy, with an average frequencies of the translocations observed for the overexposed example as much as 0.068 ± 0.012 and in the control as 0.033 ± 0.008 in the chromosome 1. A rather small dispersion between results of measurements obtained by participants of the course clearly demonstrate a reproducibility and great value of the FISH method in retrospective biological dosimetry of the absorbed dose. Results of the real investigations, performed in cells of the person presumable exposed to the radiation, based on frequency of translocations in chromosome 1, appeared to be in the range of frequencies observed in unexposed subjects, in a consequence, the risk of the deterministic overexposure to radiation have been excluded, however further study on a larger number of metaphases for investigated patient and analysis of the influence of endogenous and exogenous factors altering the cellular responses to radiation is needed to confirm our findings.