A method has been proposed for the “Hayflick Limit” prediction by the analysis of the 5-methylcytosine content in DNA at earlier and later cell passages. The following facts were used as the basis of the method: (i) the rate of m⁵C loss from DNA remains approximately constant during cell divisions and it does not depend on the cell donor age; (ii) this rate is inversely proportional to the “Hayflick Limit” as well as to the life span of cell donor species; (iii) the period corresponded to loss of all m⁵C residues from the genome coincides with or somewhat exceeds the “Hayflick Limit” of normal cells. The prognosis of the “Hayflick Limit” has usually been found in good agreement with the experimental data for various human, hamster, and murine normal and cancer cell lines. The age-related m⁵C loss may result from accumulation of the m⁵C-to-T+C transition mutations occurring with DNA methylation in every division of normal cells. In cancer cells, DNA methylation system begins to operate as if in the opposite direction increasing m⁵C content in genome. The method proposed may be used for early detection of precrisis and cancer cells.