In water (5 ml) was dissolved 7β-(D-mandelamido)-3-(3-carboxypropionyloxy)methyl-3-cephem-4-carboxylic acid (0.46 g) together with 5-mercapto-1-methyl-1H-tetrazole (0.12 g) and sodium hydrogen carbonate (0.25 g). The solution was stirred at 60° C. for 1 hour and a half. After cooling in the air, the reaction mixture was subjected to column chromatography on Amberlite XAD-2, elution being carried out with water and, then, with a solvent mixture of water and methanol. The fractions containing the desired compound are pooled, concentrated and lyophilized. The procedure provided 7β-(D-mandelamido)-3-(1-methyl-1H-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid sodium salt (0.31 g).
