A method for treatment of a bacterial infection in a host mammal in need of such treatment or a method of administering to the host mammal an effective amount of a Triazolo(4,5-d)pyrimidine of formula(I):wherein R1 is C3-5 alkyl optionally substituted by one or more halogen atoms； R2 is a phenyl group, optionally substituted by one or more halogen atoms； R3 and R4 are both hydroxyl； R is XOH, wherein X is CH2, OCH2CH2, or a bond, and wherein when X is a bond, R is OH； or a pharmaceutical acceptable salt, provided that when X is CH2 or a bond, R1 is not propyl； when X is CH2 and R1 is CH2CH2CF3, butyl or pentyl, the phenyl group at R2 must be substituted by fluorine； when X is OCH2CH2 and R1 is propyl, the phenyl group at R2 must be substituted by fluorine.
