The results of the analysis of multiaberrant cells (MAC) obtained in the course of long-term investigation of cytogenetic effects in human peripheral blood lymphocytes are presented. MAC were discovered in different groups of the population exposed to the radiation factor. No such cells were found in the control groups. The greatest number of MAC "carriers" was registered among employees of radiochemical plants who had contacts with plutonium salts. The highest frequency of MAC (2.49 +/- 0.59 per 1000 cells) was also revealed in the same group. It exceeded by an order of magnitude the analogous values in other examined groups. In the groups of radiochemical workers, cosmonauts, and miners from Tselinograd the frequency of dicentrics and centric rings was also the highest as compared to that in other groups. The character of chromosome aberrations observed in MAC suggests that they are formed under the action of the radiation factor, and their frequency among different groups of people exposed to radiation makes it possible to assume that the formation of MAC is a result of the action on lymphocytes of alpha-particles emitted by radionuclides incorporated in the organism. Classical MAC was observed in routine studies (FPG staining) are only an extreme manifestation of cell damage. To elucidate the true picture of chromosome rearrangements induced by radiation and the role of MAC in the tumor process, it is necessary to use methodical potentialities of modern molecular cytogenetics, including the FISH method.