Certain Castanospermine ester derivatives of the formula: ##STR1## wherein R, R.sub.1 and R.sub.2 are each independently hydrogen, C.sub.1-14 alkanoyl, C.sub.1-14 alkenoyl, cyclohexanecarbonyl, C.sub.1-8 alkoxyacetyl, ##STR2## naphthalenecarbonyl optionally substituted by methyl or halogen; phenyl (C.sub.2-6 alkanoyl) wherein the phenyl is optionally substituted by methyl or halogen; cinnamoyl; pyridinecarbonyl optionally substituted by methyl or halogen; dihydropyridine carbonyl optionally substituted by C.sub.1 -C.sub.10 alkyl; thiophenecarbonyl optionally substituted by methyl or halogen; or furancarbonyl optionally substituted by methyl or halogen; Y is hydrogen, C.sub.1-4 alkyl, C.sub.1-4 alkoxy, halogen, trifluoromethyl, C.sub.1-4 alkylsulfonyl, C.sub.1-4 alkylmercapto, cyano or dimethylamino; Y' is hydrogen, C.sub.1-4 alkyl, C.sub.1-4 alkoxy, halogen or it is combined with Y to give 3,4-methylenedioxy; Y" is hydrogen, C.sub.1-4 alkyl, C.sub.1-4 alkoxy or halogen; with R, R.sub.1 and R.sub.2 being selected in such a way that at least one of them, but not more than two of them, is hydrogen; are disclosed to be effective in treating diseases caused by Herpes Simplex Virus (HSV) Types 1 and 2.