Cefquinome (formerly HR 111V), an aminothiazolyl cephalosporin, was compared with cefepime, cefpirome, cefotaxime, and ceftazidime against 681 clinical cultures and a challenge set of bacteria with well-characterized resistance mechanisms. Cefquinome minimum inhibitory concentrations (MIC90) for the enterobacteriaceae ranged from < or = 0.12-2 micrograms/ml with the highest MIC (4 micrograms/ml) obtained among Citrobacter freundii, Enterobacter cloacae, and Providencia stuartii strains. A total of 90% of the Pseudomonas aeruginosa were inhibited by cefquinome at < or = 8 micrograms/ml. Cefquinome activity of particular note for Gram-positive isolates included Corynebacterium jeikeium (MIC90, 8 micrograms/ml) and enterococci (MIC50, 4-8 micrograms/ml). Oxacillin-resistant Staphylococcus aureus was 32-fold less susceptible (MIC90, 16 micrograms/ml) to cefquinome than oxacillin-susceptible (MIC90, 0.5 micrograms/ml) strains. Cefquinome was very potent against fastidious isolates such as Moraxella catarrhalis (MIC90, 0.25-2 micrograms/ml); Haemophilus influenzae (MIC90, 0.06-1 micrograms/ml), Neisseria gonorrhoeae (MIC90, 0.06-0.5 micrograms/ml), and Streptococcus species (MIC90, < or = 0.03-006 micrograms/ml). When tested against organisms possessing Bush group 2 enzymes (including extended spectrum beta-lactamases), cefquinome remained active (MIC, < or = 8 micrograms/ml) against the majority of strains. This compound should be very active against pathogens generally found in animal infections and possesses a potency and spectrum comparable to the "fourth-generation" cephalosporins (cefepime and cefpirome) being investigated for human infectious diseases.