SCE-963 {7beta-[2-(2-aminothiazol-4-yl)acetamido]-3-[({1-(2-dimethylaminoethyl)- 1H-tetrazol-5-yl}thio)methyl]-ceph-3-em-4-carboxylic acid}, a new semisynthetic cephalosporin, showed excellent antibacterial activity against gram-positive and gram-negative bacteria, including Haemophilus influenzae, indole-positive Proteus, Enterobacter species, and Citrobacter freundii. The minimum inhibitory concentrations of SCE-963 against most strains of clinically isolated Escherichia coli, Klebsiella pneumoniae, H. influenzae, and Proteus mirabilis were within the range of 0.2 to 0.78 mug/ml. These activities were about 10 times more potent than those of cefazolin, cephaloridine, and cephalothin. Variations in pH, addition of horse serum, and type of growth medium had no significant effect on the activity of the cephalosporin, but the inoculum size elicited a considerable effect on the activity of beta-lactamase-producing strains of bacteria. SCE-963 exerted bactericidal and bacteriolytic effects on Staphylococcus aureus and E. coli. The pronounced in vitro activity was reflected in the remarkable protection in mice infected with a wide range of gram-negative bacteria, such as E. coli, K. pneumoniae, P. mirabilis, Proteus vulgaris, Proteus morganii, and Proteus rettgeri. The protective effects of SCE-963 in mice infected with E. coli, K. pneumoniae, and P. vulgaris varied according to the challenge dose. The activity of SCE-963 was far more potent when the drug was administered parenterally rather than orally.