GV104326 is the first member of a new class of antibiotics (tribactams) selected for development. It combines a particularly broad spectrum (including gram-negative and gram-positive aerobes and anaerobes) with high potency, resistance to beta-lactamases, and complete stability to dehydropeptidases. Comparative MICs were determined for GV104326 against 415 recent clinical isolates (including beta-lactamase producers), using representative antibacterial agents (imipenem, amoxicillin-clavulanic acid, cefpirome, ciprofloxacin, gentamicin, and erythromycin). GV104326 was particularly active against gram-positive bacteria; in general, its in vitro activity was equivalent to that of imipenem, equivalent to or better than that of amoxicillin-clavulanic acid, and superior to that of cefpirome, ciprofloxacin, and erythromycin. Against gram-negative bacteria, GV104326 possessed activity similar to that of imipenem and cefpirome against enterobacteria and Haemophilus spp. but its activity was superior to that of amoxicillin-clavulanic acid. GV104326 showed excellent antianaerobe activity. GV104326 was stable to all clinically relevant beta-lactamases and was rapidly lethal to susceptible bacteria. In Escherichia coli, GV104326 bound predominantly to PBPs 1a and 2 and at low concentrations osmotically stable round forms were observed. GV104326 showed an affinity for PBPs 2 and 4 of Staphylococcus aureus.