The two-dimensional gel electrophoresis method of P. H. O'Farrell readily resolves approximately 1,000 proteins from whole-cell homogenates. We have found that the location of most individual proteins is sufficiently reproducible and precise to permit different laboratories to exchange information about them. We present the location of 81 Escherichia coli structural proteins, binding proteins, enzymes, and factors, identified with the aid of purified proteins supplied to us by many investigators.