The in vitro activity of N-formimidoyl thienamycin was determined against 800 gram-positive and gram-negative aerobic and anaerobic bacteria and compared with the activity of cefoxitin, cefazolin, cefamandole, cefotaxime, moxalactam, ampicillin, cefoperazone, and gentamicin. N-Formimidoyl thienamycin inhibited the majority of organisms at concentrations below 1 ,ug/ml. It inhibited methicillinresistant Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus faecalis. It inhibited ,B-lactamase-producing Haemophilus influenzae and Neisseria gonorrhoeae. Unlike other new 1-lactams, it inhibited Listeria. Escherichia coli, Klebsiella pneumoniae, enterobacters, Serratia, indole-positive Proteus, Acinetobacter, Pseudomonas aeruginosa, and Bacteroides resistant to other agents were inhibited. There was minimal effect of inoculum size and aerobic versus anaerobic conditions, and serum had no effect on activity. Most minimal bactericidal concentrations were two-or fourfold greater than the minimal inhibitory concentration. N-Formimidoyl thienamycin showed partial synergy with aminoglycosides against S. aureus, S. faecalis, and many Pseudomonas and Enterobacteriaceae. It was not hydrolyzed by plasmid-mediated and chromosomal 3-lactamases.The in vitro activity of the 1-lactam antibiotics has been markedly enlarged with recent modifications of the penicillin and cephalosporin nucleus (4, 5, 9). New penicillins and cephalosporins inhibit members of the Enterobacteriaceae, Pseudomonas aeruginosa, and anaerobic cocci and bacilli. The aminothiazolyl agents and moxalactam are hydrolyzed by certain 13-lactamases, do not inhibit enterococci, and show relatively poor activity against P. aeruginosa (4, 5). The purpose of this paper was to analyze the in vitro activity of N-formimidoyl thienamycin.The bacterial isolates used had been collected over the past 5 years because of their known resistance to one or more antibiotics and because of the presence of P-lactamases. Antimi-