CP-65,207 is a new parenteral penem antibiotic with a broad spectrum that includes gram-positive, gram-negative, and anaerobic microorganisms, with MICs for 90% (MIC90s) of the majority of 1,101 clinical pathogens tested being less than or equal to 1 microgram/ml. The compound was from 10- to 100-fold more active than cefoxitin and broad-spectrum cephalosporins against gram-positive bacteria and anaerobes. CP-65,207 was less active than imipenem for staphylococci, group A streptococci, and Enterococcus faecalis. Against members of the family Enterobacteriaceae, CP-65,207 was in general 100-fold more active than cefoxitin, 5- to 10-fold more active than broad-spectrum cephalosporins, and 2-fold more active than imipenem. Fresh clinical isolates that were resistant to broad-spectrum cephalosporins were highly susceptible to CP-65,207 and imipenem (MIC90, 1 microgram/ml). Isolates of Enterococcus faecalis, Serratia marcescens, and anaerobic Peptococcus spp. had MIC90s of 8, 2, and 3.12 micrograms/ml, respectively. CP-65,207 was not very active against methicillin-resistant staphylococci or Pseudomonas aeruginosa. Killing kinetics showed that against some strains CP-65,207 is rapidly bactericidal at concentrations well below those required to achieve a similar degree of killing with cefotaxime, ceftazidime, and ceftriaxone. CP-65,207 was only slightly susceptible to hydrolysis by type I cephalosporinases and TEM-1, SHV-1, and PSE-2 plasmid-encoded enzymes. It had the highest affinity for penicillin-binding proteins 2, 1A, 1B, and 3 in cell-free preparations of Escherichia coli W-7.