The in vitro activity of cefpirome against ceftazidime-resistant (MIC > 16 mg/l) isolates from two multicenter studies was analyzed. The first investigation carried out in the USA, was an in vitro comparison of cefpirome and five third-generation cephalosporins in which more than 6,000 isolates were evaluated, including 97 Enterobacteriaceae and 1,509 staphylococci resistant to ceftazidime. The second study was a multicenter international study (> 5,000 strains total) in which 160 ceftazidime-resistant gram-negative bacilli and 509 staphylococci from five countries (Australia, France, Germany, Italy and UK) were tested against cefpirome. The results from the US trial indicated that only 0.8% of enteric bacilli were resistant to cefpirome compared to 4.9% and 4.7% resistant to ceftazidime and cefoperazone, respectively. In the international trial, cefpirome was also active against ceftazidime-resistant, class I beta-lactamase producing enteric bacilli (75% susceptibility, MIC50 of 4 mg/l) especially against Citrobacter spp., Enterobacter spp. and Morganella morganii. Cefpirome was 8- to 64-fold more active than ceftazidime against seven different staphylococcal species. The antimicrobial activity of cefpirome against routine clinical isolates and those organisms resistant to third-generation cephalosporins was highly consistent within a nation (USA) and among various developed countries.