The in-vitro activity (as measured by geometric mean MICs, mg/l) of Sch 34343 against aerobic and anaerobic bacteria was compared with that of 14 other selected β-lactam antibiotics including aztreonam, latamoxef (moxalactam), ceftazidime and imipenem. Sch 34343 had good activity (< 2 mg/l) against most Gram-negative aerobic bacteria whether or not they contained high levels of plasmid-mediated or chromosomally-mediated β-lactamases. It was slightly less potent against strains of Morganella and Serratia (< 4 mg/l) and inactive against Pseudomonas (> 64 mg/l). A very small inoculum effect was observed against strains containing β-lactamases indicating stability. Unlike the third-generation cephalosporins, Sch 34343 had excellent activity (≤ 0.18 mg/l) against staphylococci, comparable to that of imipenem and ampicillin. While Sch 34343 had equally good potency (0.17 mg/l) against penicillinase-positive staphylococci, it was inactive against methicillin-resistant staphylococci (≥ 35 mg/l). Sch 34343 also had good activity against streptococci. The most unusual aspect of the in-vitro activity was its activity against Bacteroides (including Bact. fragilis) and other anaerobes. Sch 34343 had mean MICs ≤ 0.1 mg/l for all Bacteroides and Clostridium spp. tested except Cl. difficile (3.4 mg/l).