The in vitro activity of cefpirome was evaluated against strains that showed conflicting results for third generation cephalosporins. Against isolates with derepressed inducible chromosomal cephalosporinase (n = 40) cefpirome was the sole cephalosporin with an MIC90 in the susceptible range; Klebsiella spp. with plasmid-mediated beta-lactamases (broad spectrum SHV-2 or SHV-2 type) (n = 40) remained most susceptible to ceftizoxime and cefpirome; against aminoglycoside-resistant Pseudomonas aeruginosa (n = 50), cefpirome was as active as ceftazidime and cefoperazone; against oxacillin-susceptible and oxacillin-resistant Staphylococcus spp., (n = 40), cefpirome was more active than other third generation cephalosporins but killing was inadequate against both oxacillin-resistant staphylococci and enterococci.