Tigemonam, a new monobactam with excellent activity against gram-negative bacteria, was evaluated for in vivo efficacy and absorption after oral administration to laboratory animals. Tigemonam is absorbed when administered orally to mice and dogs. In a variety of gram-negative systemic infections in mice, orally administered tigemonam was efficacious in all infections studied. Comparison drugs such as amoxicillin, cephalexin, and cefaclor were less efficacious, especially in infections caused by beta-lactamase-producing organisms. In localized infections, tigemonam also demonstrated excellent in vivo activity. In acute pyelonephritis in mice caused by Escherichia coli or Proteus sp., tigemonam was very effective. In a rat lung model with Klebsiella pneumoniae, tigemonam was active with a median effective dose of 46 mg/kg compared with 160 mg/kg for cefaclor and over 200 mg/kg for amoxicillin. Tigemonam was well absorbed in laboratory animals and with its excellent gram-negative spectrum of activity should prove of value in oral antibiotic therapy in humans.