Minimal inhibitory concentrations (MICs) of carumonam (RO 17-2301), a new synthetic antibacterial agent of the monobactam group, were evaluated by agar dilution for 399 hospital isolates. RO 17-2301 was inactive against Gram positive and anaerobic bacteria. Most Enterobacteriaceae were inhibited by concentrations less than 1 microgram/ml, with mode MICs approximating 0.03 micrograms/ml except for Providencia (0.016), Citrobacter (0.06), Serratia (0.06) and Enterobacter (0.12). A few strains, most of which were Enterobacter or Citrobacter, had high MICs (greater than 8 micrograms/ml). RO 17-2301 was less active against Pseudomonas aeruginosa (mode MIC 2 micrograms/ml) and Acinetobacter (mode MIC 8-16 micrograms/ml). Haemophilus influenzae sp. were sensitive to RO 17-2301 (mode MIC 0.12-0.25), regardless of beta-lactamase production status; MICs ranged from 0.06 to 0.25 micrograms/ml for Meningococci, and from 0.008 to 0.06 for Gonococci except for a few strains that had higher MICs (0.25 to 0.5 and even 4 micrograms/ml). In vitro activity of RO 17-2301 on Gram negative bacteria proved similar to that of third-generation cephalosporins; cefotaxime-resistant Enterobacteriaceae are resistant to RO 17-2301.