The discovery of the monobactams led to the successful development of aztreonam as the first of this novel class of beta-lactam antibiotics to enter the clinical field. Continued structural modification on the monobactam nucleus has resulted in two additional compounds from this class that show interesting biologic properties. The first, SQ 83,360, is like aztreonam in exhibiting high activity against members of the Enterobacteriaceae but has the added characteristic of being exceptionally active against strains of Pseudomonas aeruginosa. Also, significant gains are made with SQ 83,360 in activity against Pseudomonas spp. and Acinetobacter. The second compound, tigemonam, is also like aztreonam, having good activity against Enterobacteriaceae, Haemophilus influenzae, and Neisseria gonorrhoeae and showing good beta-lactam stability. Tigemonam differs from aztreonam in being well absorbed orally by experimental laboratory animals.