To a solution of diphenylmethyl 7-(2-thienylacetamido)-3-(1-acetoxyethyl)-3-cephem-4-carboxylate (stereoisomer A: 58 mg) in methylene chloride (0.6 ml) are added anisole (0.12 ml) and trifluoroacetic acid (0.12 ml). After 15 minutes, the reaction mixture is concentrated at room temperature, and obtained residue is dissolved in ethyl acetate, and is extracted with aqueous sodium hydrogen carbonate solution. The extract aqueous solution is cooled with ice, stirred and neutralized with 1N-hydrochloric acid to pH 2, and is extracted with ethyl acetate. The extract solution is washed with water, dried over sodium sulfate, and concentrated to one third volume. To this solution is added an equivalent amount of collidine or benzylamine, and the separated crystals are collected by filtration to give 7-(2-thienylacetamido)-3-(1-acetoxyethyl)-3-cephem-4-carboxylic acid (stereoisomer A) collidine salt or benzylamine salt.
