Bacterial genomes range greatly in size, encoding from as few as 500 to more than 4000 proteins. The tools of bioinformatics have become more and more accurate at predicting open reading frames within stretches of DNA, and within this decade, the proteome complement of one or more genomes should be completed. To accomplish such a task, methods for detecting and building an inventory of the protein products encoded by the genome must be used. Such methods have been under way for over 20 years, even before the projects to complete the DNA sequence of the bacterial genome were initiated. Using two-dimensional polyacrylamide gel electrophoresis (2-D gels) (1 ), one can easily detect the proteins bacteria expressed at levels >50 mol/cell. For the bacterium Escherichia coli , about 1200 proteins can readily be detected under any one growth condition.