The activity of cefoperazone, a new broad-spectrum cephalosporin, was tested in vitro against 670 clinical isolates of gram-negative bacilli and gram-positive cocci. With the exception of Enterobacter spp., it inhibited the majority of all organisms tested at a concentration of 6.25 microgram/ml. Of particular interest is its good activity against Pseudomonas aeruginosa isolates which are usually very resistant to cephalosporins. When compared with other antibiotics, it was more active than any available cephalosporin against the Enterobacteriaceae, and its activity was comparable to the investigational drugs tested. Except against Pseudomonas, cefoperazone was less active than moxalactam (LY127935). No significant decrease in activity was noted in medium and pH variation studies. A considerable decrease in activity resulted when the size of the inoculum was incrased from 10(5) to 10(7) cells/ml. The minimal bactericidal concentrations were within one or two dilution values of the minimal inhibitory concentrations against the majority of isolates tested, except Staphylococcus aureus.