Two-dimensional (2-D) polyacrylamide gel electrophoresis has much to contribute to experimental analysis of the proteomes of microbial organisms, since this method separates most cellular proteins and allows synthesis rates to be determined quantitatively. Databases generated using 2-D gels can grow to be very large from even just a few experiments, since each sample provides the data for a field (or column) in the database for several hundreds to even thousands of records (or rows), each of which represents a single polypeptide species. The value of such databases for generating an encyclopedia of how each of the cell's proteins behave in different conditions (protein phenotypes) has been recognized for some time. The potential exists, however, to glean even more valuable information from such databases. Because the measurements of each protein are made in the context of all other proteins, a comprehensive glimpse of the cell's physiological state is theoretically achievable with each 2-D gel. By examining enough conditions (and 2-D gels), expression patterns of subsets of proteins (proteomic signatures) can be found that correlate with the cell's state. This type of information can provide a unique contribution to proteomic analysis, and should be a major focus of such analyses.