Viral infections caused by viruses which belong to, e.g. Pox, Herpes, Adeno, Myxo, Paramyxo groups are controlled by administering an effective amount of quinazoline derivatives of the formula ##STR1## wherein R.sub.1 is cyclo, C.sub.3 -C.sub.8 alkyl, cyclo C.sub.3 -C.sub.8 alkyl C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy C.sub.1 -C.sub.4 alkyl, C.sub.2-C.sub.5 alkenyloxy C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkylthio C.sub.1 -C.sub.4 alkyl, polyhalo C.sub.1- C.sub.4 alkyl, a group of the formula ##STR2## (wherein m is an integer of 1 to 3; and R.sub.4 and R.sub.5 are individually C.sub.1 -C.sub.4 alkyl, and may form together with the adjacent nitrogen atom an unsubstituted or C.sub.1 -C.sub.4 alkyl substituted 3 to 6 membered saturated heterocyclic ring, which may contain another nitrogen or oxygen atom) or a group of the formula --C.sub.n H.sub.2n --R.sub.6 (wherein n is 0 or an integer of 1 to 3; and R.sub.6 is a phenyl group, a substituted phenyl group or an aromatic or non-aromatic 3 to 6 membered heterocyclic ring which contains one or two hetero atoms selected from the group consisting of nitrogen, oxygen and sulfur atom, and may be optionally substituted by one or two C.sub.1 -C.sub.4 alkyl, and further, those substituents on adjacent carbon atoms may be joined to form a benzene or cyclohexane ring); R.sub.2 is hydrogen, halogen, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy, C.sub.1 -C.sub.4 alkylthio, C.sub.1 -C.sub.4 alkylsulfonyl, trifluoromethyl or nitro; and R.sub.3 is phenyl, substituted phenyl, cyclo C.sub.3 -C.sub.6 alkyl, pyridyl, furyl, nitrofuryl, thienyl, nitrothienyl, methylthienyl or pyrimidinyl.