Abstract: Applicants have developed a process for purifying an ampicillin pro-drug ester, e.g., bacampicillin, or an acid addition salt thereof of formula I. The process comprises a step wherein a crude solution of the pro-drug ester is subjected to an evaporation which is controlled by the evaporation rate of the organic solvent such that a continuous operation is accomplished for the crystallization of the desired purified pro-drug ester or salt thereof. Said crystallization is preferably preceded by an aqueous phase extraction and/or an organic phase extraction, both preferably also being performed continuously.