The in vitro qualities of cefoperazone were reviewed on the basis of the international medical literature and some new observations. Cefoperazone is highly active against the Enterobacteriaceae. Its activity against Staphylococcus aureus is comparable to that of the other newer cephem antibiotics. Cefoperazone is also active against all β-hemolytic streptococci and Streptococcus pneumoniae and is relatively inactive against methicillin-resistant S. aureus and enterococci. Against Pseudomonas aeruginosa cefoperazone is at least fourfold more active than cefotaxime or moxalactam and is approximately as active as azlocillin or piperacillin. Haemophilus and Neisseria species, regardless of β-lactamase production, are highly susceptible to cefoperazone. Against the Bacteroides jragilis group, cefoperazone is either very active or quite inactive because of endemic variations. The drug is slightly less stable to some β-lactamases than are cefotaxime-like or 7-methoxy cephem drugs. Cefoperazone is a bactericidal β-lactam, and its minimal inhibitory concentrations are influenced only by high inoculum concentrations of β-lactamase-producing strains. Its ability to permeate bacterial cell membranes appears similar to that of cefotaxime. Synergy studies with cefoperazone plus β-lactamase inhibitors or aminoglycosides against Enterobacteriaceae and P. aeruginosa show enhanced killing. Cefoperazone is 70%–94% protein bound and has high affinities for bacterial penicillin-binding proteins 3, 1 a, 2, and 1 bs.