A cytogenetic investigation of Vero cells, before and after adaptation to the medium containing a cattle serum, was carried out by methods of differential chromosome staining. Under these conditions, both the modal number of chromosomes (from 58 to 55) and the karyotype structure, namely the copy number of normal chromosomes and the marker composition were shown to change. The Vero cell karyotype stability was studied in the continued culture by the static (50 passages) and roll-bottle (37 passages) methods. The quantitative changes (the rising percentage of diploid cells, and the change of cell fraction involving the modal number of chromosomes) were shown to occur in spite of the chromosome composition stability, which limits the time of using Vero cells as a substrate for preparation of vaccines.