Benzoylpiperazine esters and acid addition salts thereof are disclosed, which esters have the following formula:
wherein A represents a single bond or an alkylene group, vinylene group, -O-alkylene group or methine group, R 1 represents a bicyclic carbon ring residue which may be substituted with a lower alkyl group, lower alkoxy group, oxo group or nitro group or a halogen atom, or may be partially saturated; a fluorene residue which may contain an oxo group; a fluorenylidene group; an anthracene residue; a phenathrene residue which may be substituted with a lower alkyl group, or may be partially saturated; a benzofuran residue or thianaphthene residue which may be substituted with a lower alkyl group or lower alkoxy group; a benzopyran residue or benzoazine residue which may be substituted with an oxo group or phenyl group and partially saturated; a phthalimide residue; a benzodiazine residue; an isooxazole residue which may be substituted with a lower alkyl group or phenyl group; or an alkylene dioxybenzene residue or xanthene residue; and R 2 represents an alkyl group, cycloalkyl group, cycloalkylalkyl group or aralkyl group; except the case where A is a single bond, R l is
and R 2 is a methyl group. The esters exhibit excellent chymotrypsin inhibitive activity.