Abstract: The compound 2-.beta.-D-ribofuranosylthiazole-4-carboxamide is used to treat malignant tumors in warm blooded animals. Esters of this compound such as 2-(2,3,5-tri-O-acetyl-.beta.-D-ribofuranosyl)thiazole-4-carboxamide and 2-(5-O-phosphoryl-.beta.-D-ribofuranosyl)thiazole-4-carboxamide are also useful for treating tumors in warm blooded animals.