Ceftezole, a new cephalosporin antibiotic similar to cefazolin, has the following chemical structure: (6R,7R)-8-oxo-7[2-(1H-tetrazol-1-yl)acetamido]-3-[(1,3,4-thiadiazol-2-ylthio)methyl]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-carboxylic acid. Ceftezole was found to be a broad-spectrum antibiotic, active in vitro against many species of gram-positive and gram-negative bacteria except Pseudomonas aeruginosa, Serratia marcescens and Proteus vulgaris. The activity of ceftezole against clinical isolates of Escherichia coli and Klebsiella spp. appeared to be nearly equal to that of cefazolin and higher than those of cephaloridine and cephalothin. Cross-resistance was observed between ampicillin and cephaloridine, but not between ampicillin and ceftezole, in susceptibility tests on clinical isolates of P. mirabilis. The in vitro activity was little affected by the inoculum size, the presence of human serum or the test medium. Ceftezole exhibited apparent bactericidal activity at the concentrations above the minimum inhibitory concentration (MIC) against both S. aureus and E. coli. The development in vitro of resistance by S. aureus 209p and E. coli NIHJ to ceftezole after 16 transfers was similar to or somewhat slower than that to other drugs tested. Ceftezole was relatively stable in nutrient broth and minimally degraded in the serum or tissue homogenates of rats. Ceftezole, in a single subcutaneous administration, exhibited somewhat less efficacy in mice against intraperitoneal infections with Streptococcus pyogenes, S. pneumoniae, E. coli, K. pneumoniae or P. mirabilis than either cephaloridine or cefazolin. However, ceftezole exhibited efficacy similar to that of cephaloridine or cefazolin when administered in three doses. Furthermore, ceftezole was as effective as cefazolin in the treatment of experimental abscesses in mice caused by subcutaneous inoculation with S. aureus.