Abstract Cefoperazone was tested against 554 clinical isolates alone and with sulbactam in three combinations. The addition of sulbactam in low concentrations (≤4 µg/ml) improved the spectrum of cefoperazone principally against gram-negative bacilli such as Acinetobacter species, some Pseudomonas species, and β-1actamase-positive Enterobacterioceae. Nearly all of the spectrum increase was achieved at a sulbactam level of ≤2 µg/ml. Sulbactam was found to be an effective antimicrobial agent agains Acinetobacter species (MIC 50, 1.0 µg/ml), Pseudomonas acidovorans (MIC 50, 2.0 µg/ml), Neisseria gonorrhoeoe (MIC 50, ≤0.5 µg/ml), and N. meningitidis (MIC 50, ≤0.5 µg/ml). Sulbactam had a higher affinity and binding constant for the plasmid-mediated β-lactamases such as TEM-1 and TEM-2 compared to cefoperazone (≥lO-fold difference). This finding was important as cefoperazone con be hydrolyzed at a moderate rate by the highly efficient TEM enzymes (<2% of clinical Escherichia coli isolates). Sulbactam increased the susceptibility (≤16 µg/ml) of 220 isolates of Enterobacteriaceoe to cefoperazone from 88.6 to 96.3% when 4.0 µg/ml of sulbactam was added. The cefoperazone antimicrobial activity was also increased against the nonenteric bacilli from a 69.5 to a 87.4% total inhibition. MICs among cefoperazone-susceptible gram-negative and gram-positive strains were rountinely decreased 2- to 32-fold, as calculated from MIC 90 results. Therefore, sulbactam should predictably increase the antimicrobial spectrum and clinical effectiveness of cefoperazone against nosocomial and other pathogens such as the plasmid-containing enteric bacilli, Bacteroides species and Acinetobacter species, and possibly provide the opportunity to reduce dosage schedules for infecting species already susceptible to cefoperazone alone.