Karyotypic destabilization in cells of Chinese hamster fibroblasts CHL V-79 RJK with ectopically overexpressed antiapoptotical human bcl-2 gene from pSFFV-bcl-2 vector has been analysed. Analysis of G-banded metaphase chromosomes from 4 clones with different levels of bcl-2 expression revealed an increased level of chromosomal instability in bcl-2-transfected cells. Besides, an increased percentage of aneu- and polyploid cells and high level of cells with different chromosomal aberrations was observed. The degree of karyotypic instability positively correlated with the level of bcl-2 expression in bcl-2-transfected cells. Cells of a clone with the highest bcl-2 expression at the 13th passage of cultivation displayed an almost 100% polyploidization and the presence of specific aberrations and a tricentric marker chromosome. Selection of cells with non-random specific chromosome changes was observed in pSFFV-bcl-2-transfected CHL V-79 RJK cells in the process of their long-term cultivation. By contrast, cells of the parental cell line, as well as the control pSFFV-neo transfectants, displayed a stable karyotype throughout the long period of cultivation. It is important that the presence of morphological markers of gene amplifications--DOO, DM, MH--was observed in bcl-2-transfected cells. These findings suggest that the overexpression of antiapoptotic human bcl-2 gene may result in destabilization of the karyotype structure in cells of Chinese hamster fibroblasts CHL V-79 RJK. The character and level of destabilization correlate with the level of ectopic overexpression of this gene.