New cephalosporin antibiotics, 7-[(3,4-dihydro-6-methoxycarbonyl-2,2-dimethyl-2H-1,4-thiazin-3-yl)acetamido]-3-substituted-3-cephem-4-carboxylic acid derivatives 2a-2d, 7 -[(3,4-dihydro-6-ethoxycarbonyl-2,2-dimethyl-2H-1,4-thiazin-3-yl)acetamido] -3-substituted-3-cephem-4-carboxylic acid derivatives 3a-3d and 7-[(3,4-dihydro-6-methoxycarbonyl-2,2-dimethyl-2H-1,4-thiazin-3-yl-1-(S)-oxide)acetamido]-3-substituted-3-cephem-4-carboxylic acid derivatives 4a-4d were synthesized. Antibacterial activities of these new cephalosporin derivatives and the relationship between their structures and their activities were examined. Among them, 7-[(3,4-dihydro-6-methoxycaronyl-2,2-dimethyl-2H-1,4-thiazin-3-yl-1-(S)oxide)-acetamido]-3-[(1,2,3-triazol-5-yl)thiomethyl]-3-cephem-4-carboxylic acid 4d exhibited the antibacterial activities against Gram(+)and Gram(-) bacteria.