The present invention provides 2.beta.-chloromethyl-2.alpha.-methylpenam-3.alpha.-carboxylic acid benzhydryl ester (CMPB) crystals of excellent stability, and a process for producing the CMPB crystals comprising the steps of (A) concentrating a solution containing CMPB; (b) subjecting the thus-obtained concentrate to column chromatography; (C) concentrating a CMPB-containing fraction; and (D) dissolving the thus-obtained CMPB-containing concentrate in an ether solvent and adding a hydrocarbon solvent to the resulting solution to precipitate CMPB crystals.