Antibacterial derivative of 7-aminocephalosporanic acid

N - (7 - 21 - thienylacetami doceph - 3 em -3 - yl methyl) - pyridinium - 4 - carboxylate is prepared by reacting a compound of formula where R is H or a 2-thienylacetyl, with pyridine in a strongly polar medium, the resulting compound where R is H, being 2-thienylacetylated. Alternatively, cephalosporin C may be treated with pyridine, converted to the 7-ACA derivative, which is then 2-thienylacetylated. Compound IIa may be in the form of a water-soluble salt.ALSO:A pharmaceutical formulation for human or veterinary use comprises N-(7-21-thienyl-acetamidoceph-3-em-3yl methyl)pyridinium-4-carboxylate. The compositions may be for oral, parenteral or topical use and include a veterinary arate. Optional ingredients are antibacterial antibiotics such as penicillins and/or tetracyclines.