Cultured HEp-2 cells (human larynx cancer cells) reveal significant variation in chromosome aberration rate. The study of its regularities at cell and population levels allowed to conclude that the cancer cells contain hidden lesions of genetic structures which, with certain probability, can reveal themselves as chromosome aberrations. Exposure of cancer cells to non-mutagenic agents (suboptimal temperature, low concentration of propilgallate or caffeine) resulted in the increase of frequency of cells containing chromosome aberrations above the control level, while the number of aberration per aberrant cell was not altered. Exposure of irradiated cells to the same agents induced, as in the case of spontaneous mutagenesis, the increase of the aberrant cell frequency, while the extent of their damage remained at the level characteristic of the effect of irradiation solely. The similarity of mechanisms for realisation of hidden chromosome lesions under the action of non-mutagenic agents in spontaneous and radiation-induced mutagenesis indicates the similarity of the nature of these lesions. The convincing evidence of existence of hidden genetic lesions in cancer cells was obtained in experiments with cloning of the initial population and analysis of chromosome aberrations in 22 clones. All the cells without any exception contained chromosome aberrations.