New cephalosporin antibiotics,7-[(3,4-dihydro-6-methoxycarbonyl-2,2-dimethyl-2H-1,4-thiazin-3-yl)acetamido]-3-[(substituted pyrimidin-2-yl)thiomethyl]-3-cephem-4-carboxylic acid derivatives(2a∼2d) were synthesized. These new cephalosporin derivatives were prepared by the introduction of pyridinylthiomethyl moiety in 3-position and thiazine group in 7-position of 7-ACA. Antibacterial activities of these compounds were examined and the relationship between structure and activities were studied. As the result, these compounds showed low antibacterial activities compared to cefotaxim used as control.