Genes similar to Escherichia coli lpx genes (encoding enzymes required for the biosynthesis of lipid A) have been cloned from Haemophilus influenzae type b using a hybridisation-based strategy. The derived amino acid sequences are highly homologous to their E. coli counterparts. The genes appear in the same order in both E. coli and H. influenzae, but the intergenic regions differ. H. influenzae lpxA and lpxB have been expressed in E. coli minicells and they encode proteins of the predicted sizes. Both H. influenzae lpxA and lpxB are able to complement temperature-sensitive mutants in the equivalent genes in E. coli. This provides evidence that the genetic manipulation of lpx genes to generate altered lipid A molecules may be possible.