An antitumor drug 6-azacytidine, being an inhibitor of nucleic metabolism, has a broad spectrum of the antiblastic action. During the study of its pharmacological properties it proved to be low toxic, to exert a mild action on hemopoiesis, to fail to suppress the liver antitoxic function, to have a high chemotherapeutic index and a broad spectrum of the therapeutic action, to exert no local irritant effect on the skin and mucosae of the eye, to possess a sufficiently high reversibility of the toxic action.