Cell hybrids obtained by fusing mouse PCC4aza1 teratocarcinoma cells and spleen cells induced to proliferation and treated with the demethylating agent 5-azacytidine prior to fusion are described. The obtained hybrids demonstrated no expression of T lymphocyte marker genes CD11 and CD45, which indicates possible somatic nucleus reprogramming by factors present in teratocarcinoma cells. Irrespective of culture conditions, cell hybrids demonstrated a relatively stable chromosome number: they lost on average no more than four chromosomes after 30 passages. Culturing in medium containing hypoxanthine, aminopterin, and thymidine (selective conditions) decreased the differentiation capacity of cell hybrids compared to nonselective conditions, which is likely due to the inhibition of their metabolism. For the first time, teratocarcinoma cell hybrid differentiation into cardiomyocytes under the influence of DMSO has been demonstrated in vitro. © 2008 MAIK Nauka.