The in-vitro activity of SQ 26,776, (SQ)a novel monocyclic β -lactam compound was determined against 600 bacteria and compared to that of cefotaxime, moxalactam, cefoperazone, piperacillin, and tobramycin. SQ did not inhibit Gram-positive species or anaerobic species-streptococci, staphylococci, Clostridia. It inhibited 90% of E. coli , Kiebsiella , Salmonella , Providencia , Shigella , Citrobacter , and Morganella by ≤0.4. This was equivalent to cefotaxime and moxalactam, and superior to cefamandole, cefoxitin, cefoperazone, and piperacillin. SQ was as active as cefotaxime, ceftizoxime and moxalactam against Serratia, Ent . cloacae and Ent. aerogenes. SQ inhibited Ps. aeruginosa resistant to piperacillin, cefoperazone, cefsulodin, ceftazidime and tobramycin—all strains were inhibited by 25 mg/l and 75% by 6 mg/l. Acinetobacter and Ps. maltophilia were not inhibited. Type of or pH of medium, inoculum size and aerobic or anaerobic conditions did not alter MIC or MBC values. SQ was stable to plasmid and chromosomal β -lactamases of TEM 1,2, SHV, Oxa P99 and PSE types.