Abstract: 7-[D-(.alpha.-Amino-.alpha.-phenyl-, 2-thienyl- and 3-thienylacetamido)]-3-(thiazol-2-yl)carbonylthiomethyl-3-cephem-4-carboxy lic acid and 7-[D-(.alpha.-amino-.alpha.-phenyl-, 2-thienyl- and 3-thienylacetamido)]-3-(5-methylthiazol-2-yl)carbonylthiomethyl-3-cephem-4 -carboxylic acid and their pharmaceutically acceptable salts are valuable as antibacterial agents, as nutritional supplements in animal feeds, as agents for the treatment of mastitis in cattle and as therapeutic agents in poultry and animals, including man. The compounds are especially useful in the treatment, particularly by oral administration, of infectious diseases caused by many Gram-positive and Gram-negative bacteria. Also included within the invention are easily cleavable esters of the above acids and pharmaceutically acceptable acid addition salts of said esters.