A new semisynthetic cephalosporin antibiotic designated 7-β-(1-benzylpyridinium-4-yl)-amino-3-{[(1-methyl-1H-tetrazol-5-yl)thio]methyl)ceph-3-em-4-carboxylate (L-640, 876) was compared for antibacterial activity in vitro with mecillinam, cefoxitin and cefotaxime. The antibacterial spectrum of L-640, 876 and the effect of culture medium composition and inoculum size on activity are most similar to those of mecillinam. In some cases the inoculum effect on MICs correlated with instability of the compound to certain β-lactamases and in others to the presence of ionized compounds such as sodium chloride in the medium. On balance, L-640, 876 was superior to mecillinam in potency and breadth of spectrum.