The bacterial proteome is the total protein complement expressed from the genome under defined conditions of growth. Unlike the genome, the proteome is not a fixed characteristic, but varies with the conditions of growth of the organisms from which proteins are harvested. Proteomics gives both qualitative data on the physical characteristics of the proteins and quantitative data on the synthesis of the individual proteins. These data provide a global overview of the variability of proteins among bacterial isolates as well as allowing global changes in protein synthesis (reflecting changes in gene expression) to be monitored in response to environmental stimuli. The proteomes for a number of bacteria are currently under investigation, in each case the studies benefiting from the extensive genome sequence data currently available. Several researchers have also published extended studies on the analysis of the Haemophilus influenzae proteome (1–5).