New cephalosporin antibiotics. 7-[(1H-tetrazolyl)acetamidol]-3-[(4-carboxy-3-hydroxyisothiazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid trisodium salt 2,7-[(5-ethylthio-3-hydroxyisothiazol-4-yl)carbox-amidol-3-[(4-carboxy-3-hydroxyisothiazo1-5-yl)thiomethyl)-3-cephem-4-carboxylic acid 3,7-[(2-aminothiazol-4-yl)acetamido]-3-[(4-carboxy-3-hydroxyisothiazol-5-yl]thiomethyl]-3-cephem-4-caboxylic acid 4,7-[(Z)-2-(2-aminothiazol-4-yl)-2-(alkoxyimino)acetamido)-3-[(4-carboxy-3-hydroxyisothiazol-5-yl)thiomethyl)-3-cephem-4-carboxylic acid 5-9 were synthesized. Antibacterial activities of these new cephalosporin derivatives and the relationship between their structures and activities were examined. Among them, 7-((Z)-2-(2-aminothiazol-4-yl)-2-(carboxymethoxyimino)acetamido)-3-[(4-carboxy-3-hydroxyisothiazol-5-yl)thiomethyl)-3-cephem-4-carboxylic acid 8 and 7-[(Z)-2-(2-aminothiazol-4-yl)-2-(l-carboxy-1-methylethoxyimino)acetamido]-3-[(4-carboxy-3-hydroxyisothiazol-5-yl)thiomethyl)-3-cephem-4-carboxylic acid 9 exhibited good antibacterial activities compared with those of cefotaxime and ceftriaxone.