2-Ethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)-acetic acid (syn isomer)(11.4 g) and 1-[(7-amino-4-carboxy-3-cephem-3-yl)methyl]pyridinium chloride hydrochloride dihydrate (24 g) were reacted according to a similar manner to that of Example 1. To the reaction mixture was added water (300 ml) and the mixture was adjusted to pH 1.5 with an aqueous solution of sodium hydroxide and subjected to column chromatography on a non-ionic adsorption resin Diaion HP-20 (960 ml). After the column was washed with water (3.6 l) and eluted with 30% aqueous methanol. (1.26 l). The eluates were collected and concentrated under reduced pressure to a volume of 110 ml. The solution was subjected to column chromatography on acidic alumina (120 g) and eluted with water. The eluates containing 7-[2-ethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)acetamido]-3-(1-pyridiniomethyl)-3-cephem-4-carboxylate (syn isomer)(310 ml) were collected and acetone (1.1 l) was added thereto with stirring at ambient temperature for 5 minutes. The mixture was stirred for 1.5 hours under ice-cooling and precipitates were collected by filtration, in turn washed with 90% aqueous solution of acetone (80 ml) and acetone (240 ml), and then dried under reduced pressure to give crystals (20 g) of 7-[2-ethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)acetamido]-3-(1-pyridiniomethyl)-3-cephem-4-carboxylate acetone adduct (syn isomer). The crystals were dissolved in water (40 ml) and the solution were passed through a column packed with acidic alumina (40 g). The elution was carried out with water and the eluates (140 ml) were collected. To the solution was added acetone (300 ml) at ambient temperature under stirring, which was stopped in a minute. The mixture was stood for 20 minutes at ambient temperature and for 1.5 hours in a refrigerator. The resulting colorless needles were filtered, washed with 90% aqueous acetone (25 ml×3) and acetone (25 ml×3) and dried for 3.5 hours under reduced pressure to give pure colorless needles of 7-[2-ethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)acetamido]-3-(1-pyridiniomethyl)-3-cephem-4-carboxylate acetone adduct (syn isomer)(14.3 g), mp>157° C.
