Crystalline 7-[2-(2-aminothiazol-4-yl)-2-hydroxyiminoacetamido]-3-vinyl-3-cephem-4 -carboxyli c acid (syn isomer) which is obtainable by acidifying a solution containing 7-[2-(2-aminothiazol-4-yl)-2-hydroxyiminoacetamido-3-vinyl-3-cephem-4- carboxylic acid (syn isomer) at room temperature or under warming. Such crystalline compound shows a characteristic X-ray diffraction pattern. The invention comprises also processes for preparing such crystalline compound and pharmaceutical compositions containing it as active ingredient.
