The resolution of proteins from whole-cell homogenates by two-dimensional gel electrophoresis is sufficiently reproducible and precise to permit different laboratories to exchange information about them. To the previous total of 81 we add the locations of 55 Escherichia coli proteins determined with the aid of purified proteins and mutant strains supplied by many investigators. The criteria used to establish the identifications of protein spots include migration with marker proteins, altered position or amount in appropriate mutant or plasmid-carrying strains, physiological behavior, and peptide map pattern.