Heterocyclyl derivatives of oxy-imino-substituted cephalosporins are disclosed, such, for instance the compound 3-[(8-amino-6-tetrazolo[1,5-b]pyridazinyl)-thiomethyl]-7-[2-(2-amino-4-thi azolyl)-2-methoxyimino-acetamido]-3-cephem-4-carboxylic acid and its pharmaceutically and veterinarily acceptable salts. The compounds of the application have high antibacterial activity against Gram-positive and Gram-negative bacteria, including strong beta-lactamase producer Gram-negative microorganisms.