Abstract: Novel diacyl penicillins of the formula ##EQU1## wherein R is alkyl, cycloalkyl, aryl, aralkyl, phenoxyalkyl, heterocyclic carboxyl excepting isoxazole carboxyl derivatives, or ##SPC1##In which benzene ring A may optionally be substituted andB represents a protective group for the amino group, and X is a protective group for the carboxyl group, are produced by reacting a benzyl penicillin ester of the formula ##EQU2## wherein X is as defined above, with a chlorinating agent in the presence of tertiary organic base to obtain an imide chloride group-incorporated compound of the formula ##EQU3## and then reacting the compound of the last-named formula with carboxylate of the formulaR-COOM (IV)wherein M is a metal atom, and R is as defined above.