The in vitro activity of LJC 10,627, a new carbapenem, was compared with those of imipenem, cefotaxime, ceftazidime, and gentamicin. LJC 10,627 inhibited 90% of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae, Hafnia alvei, Citrobacter freundii, Citrobacter diversus, Proteus mirabilis, Morganella morganii, Proteus rettgeri, Serratia marcescens, Pseudomonas cepacia, salmonellae, shigellae, aeromonas, and yersiniae at less than or equal to 2 micrograms/ml. Haemophilus influenzae was inhibited by 0.5 microgram/ml, and moraxellae were inhibited by 0.12 microgram/ml. LJC 10,627 was twofold more active than imipenem against aerobic gram-negative organisms and inhibited ceftazidime-, cefotaxime-, and gentamicin-resistant members of the genera Klebsiella, Enterobacter, Citrobacter, and Serratia at less than or equal to 2 micrograms/ml. Xanthomonas maltophilia strains were resistant to the drug. Imipenem was two- to fourfold more active than LJC 10,627 against Staphylococcus aureus and Staphylococcus epidermidis. LJC 10,627 did not inhibit most methicillin-resistant Staphylococcus aureus or methicillin-resistant Staphylococcus epidermidis strains. LJC 10,627 inhibited Streptococcus pyogenes and Streptococcus pneumoniae at 0.06 and 0.12 microgram/ml, respectively. Bacteroides fragilis and other Bacteroides spp. were inhibited by 0.5 microgram of LJC 10,627 per ml. Serum (50%) did not affect the MICs. LJC 10,627 was not hydrolyzed by plasmid-mediated beta-lactamases of Bush types 2b, 2b', TEM-1, TEM-2, TEM-3, TEM-5, TEM-7, TEM-9, and SHV-1; the chromosomal beta-lactamases of Bush type 1; P-99; a Morganella enzyme; or a Citrobacter freundii enzyme. The Bush type 2c and 2d enzymes OXA-1, OXA-2, PSE-1, PSE-2, and PSE-4 did not hydrolyze LJC 10,627, nor did the beta-lactamases of Staphylococcus aureus, Moraxella spp., Bacteroides fragilis, and Proteus vulgaris. The beta-lactamase of Xanthomonas hydrolyzed LJC 10,627, albeit at approximately one-third the rate that imipenem was hydrolyzed.