New 7- (2- (2-aminothiazol-4-yl) -2-hydroxyiminoacetamido) -3-vinyl-3-cephem-4-carboxylic acid (syn isomer).

ACID CRYSTALLINE 7 - (2 - (2 - aminothiazol - 4 - IL) - 2 HIDROXIAMINOACETAMIDO) - 3 - VINIL - 3 - CEFEM - 4 - carboxylic acid (SYN isomer) which is obtained by acidifying a solution containing 7 - (2 - ( 2-aminothiazol - 4 - IL) - 2 - HIDROXIAMINOACETAMIDO - 3 - VINIL - 3 - CEFEM -. 4 - carboxylic acid (SYN isomer) at room temperature or warm AS shows a drawing crystalline compound characteristic diffraction X-ray THE INVENTION It also includes processes for preparing such crystalline compound and pharmaceutical compositions containing it as active ingredient.