On the basis of the knowledge that the amino acid 3 (8-amino-2,6-anhydro-3,8-dideoxy-D-glycero-D-talo-octonic acid) is a potent inhibitor of 3-deoxy-manno-octulosonate cytidylyltransferase, attempts were made to design derivatives that would act as antibacterials against Gram-negative bacteria by inhibiting lipopolysaccharide biosynthesis. Compound 3 and the derivatives 15 and 16 containing an additional amino acid were not lethal to bacteria. However, compounds 17-22, which contain a N-terminally linked dipeptide, exhibited good antibacterial activity in vitro on testing against strains of the Gram-negative bacteria Escherichia coli and Salmonella typhimurium. They have no activity against Gram-positive bacteria such as Staphylococcus aureus.