The in vitro activity of tigemonam, a new oral monobactam, was studied with special attention to minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs). Against 250 clinical isolates, it inhibited 100% of Escherichia sp., Klebsiella sp., Serratia sp., Citrobacter sp., Providencia sp., Proteus sp., Morganella sp., Aeromonas sp., Yersinia sp., Shigella sp., and Haemophilus sp. at 0.5 mg/L or less. With 1 mg/L, 75% of Enterobacter sp. were inhibited; however, three of the 20 strains tested needed more than 16 mg/L. Proteus sp., Morganella sp. and Providencia sp. were more susceptible, with MIC90s of 0.06 mg/L or less. The MBC was equal to or two times higher than the MIC. Increasing the inoculum size from 10(3) to 10(5) colony-forming units had little effect on MIC and MBC; with an inoculum of 10(7) or more, MIC and MBC increase three to eight times. MIC and MBC were a little lower in Mueller-Hinton base, and the presence of serum did not significantly change the MIC or the MBC. Tigemonam exhibits a rapid killing rate, but an increased antibiotic concentration was not accompanied by greater lethal effect, and the lethal rate at MBC was lower in trypticase soy broth (TSB) + 40% serum than in TSB alone.