A thermostable protein complex (TSPC) obtained from the myocardium ventricules of adult rats inhibits mitotic and transcriptional activity of cardiomyocytes. At the same time this complex is more active on early stages of the postnatal ontogenesis in rats, aged 1 and 5 days. Following its action on RNA synthesis, the TSPC reveals tissue specificity only in cells with terminal differentiation, and is determined by its nuclear membranes. We continue studies for identifying the molecular weight and chemical nature of the TSPC, and the role of its different fractions in regulation of proliferation processes. Besides, it is planned to produce antibodies against TSPC fractions with the purpose to block its inhibitory effect on myocyte regeneration in the damaged myocardium.