Abstract: 2-Dimethylamino-6-diethyleneimidophosphamido-7-methylpurine having the formula ##STR1## A method for preparing 2-dimethylamino-6-diethyleneimido-phosphamido-7-methylpurine consisting of treating theo-bromine with heating to the boiling point and subsequently with phosphorus oxychloride and phosphorus pentachloride. The prepared 2,6-dichloro-7-methylpurine is heated to 50.degree. - 60.degree. C in an aqueous solution of ammonia and the resulting 2-chloro-6-amino-7-methylpurine is reacted, with heating, with a aqueous solution of dimethylamine. The obtained 2-dimethylamino-6-amino-7-methylpurine is reacted, at the boiling point, with either phosphorus oxychloride, or phosphorus pentachloride and formic acid. The resulting 2-dimethylamino-7-methylpurinyl-6-aminophosphoric dichloroanhydride is then treated with ethyleneimine in a medium of an organic solvent in the presence of an acceptor of hydrogen chloride, and the final product is then isolated.