In a mixture of 4 ml of formamide and 4 ml of acetonitrole was suspended 423 mg of sodium 7β-[2-(2-aminothiazol-4-yl)acetamido]-3-hydroxymethyl-3-cephem-4-carboxylate.dihydrate. To the suspension were added in sequence 315 mg of 1-(2-dimethylaminoethyl)-5-mercapto-1H-tetrazole hydrochloride, 0.45 ml of trimethyl phosphite and 0.20 ml of ether solution of hydrogen chloride (5 mol./l). The mixture was stirred at 20~25° C. for 1.5 hour, and there was added 5 ml of water, followed by concentration under reduced pressure at 20 to 25° C. To the concentrated solution was added 50 ml of water, whose pH was adjusted to 5.8 with 1N NaOH, followed by subjecting the resultant to an Amberlite XAD-2 column chromatography, eluting with a mixture of water and acetone (4:1,v/v). Fractions containing the end product were combined, concentrated under reduced pressure and then freeze-dried to give 7β-[ 2-(2-aminothiazol-4-yl) acetamido]-3-[1-(2-dimethylaminoethyl)-1H-tetrazol-5-yl]thiomethyl-3-cephem-4-carboxylic acid.
