Abstract: The invention relates to a novel, heretofore unknown polymorphous modification of 5-fluoro-1-(tetrahydro-2-furyl)uracil (tegafur) having antineoplastic activity. This form is crystalline, and its characteristics it differs essentially from the modification known earlier. In particular, it has an enhanced specific activity. The new form is physically stable and may find application in medicine for treating oncological patients. New anticancer medicinal substances prepared in the form of stable molecular complexes on the basis of said new modification, in particular, crystalline complexes with 6-methyluracil, and amorphous complexes with the biologically active substances from licorice, are also described.