The sodium salt of 3-acetoxymethyl-2-carboxy-8-oxo-7-[(1,3-dithiol-2-on-4-yl)-acetamido]-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (6.1 g.) is dissolved in water (20 cc.). Potassium thiocyanate (26.6 g.), sodium bicarbonate (1.38 g.) and 2-methyl-5-thioxo-1,2,3,4-tetrazoline (1.9 g.) are added to this solution and the mixture is heated to 60° C. for 5 hours. After cooling, the reaction mixture is diluted with water (100 cc.), acidified to pH 5.8 by adding 4 N hydrochloric acid and washed with ethyl acetate (50 cc.). The aqueous phase is acidified to pH 1.6 by adding 4 N hydrochloric acid in the presence of ethyl acetate (200 cc.). After decanting the organic phase, the aqueous phase is extracted twice with ethyl acetate (a total of 200 cc.). The organic extracts are combined, washed twice with an 0.1 N hydrochloric acid solution (a total of 140 cc.) and then twice with water (a total of 200 cc.), dried over sodium sulphate, treated with decolourising charcoal, filtered and then concentrated to dryness under reduced pressure (20 mm Hg). The residue obtained is dissolved in tetrahydrofuran (25 cc.) and this solution is then added dropwise to isopropyl ether (300 cc.). A product precipitates which is isolated by filtration and dried under reduced pressure (0.1 mm Hg). 2-Carboxy-3[(2-methyl-1,2,3,4-tetrazol-5-yl)-thiomethyl]-8-oxo-7-[(1,3-dithiol-2-on-4-yl)-acetamido]-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (5.1 g.) is thus obtained.
