The metabolism of 184 individual proteins was examined in exponentially growing Escherichia coli. Cells were labeled with [3H]leucine and [35S]methionine using either a pulse-chase or a continuous labeling method. Proteins were fractionated by two-dimensional gel electrophoresis and their stabilities relative to total protein were determined from the 3H/35S ratios. Forty-seven proteins appeared to be unstable and were probably either degraded or modified to electrophoretically distinct forms. The apparent half-lives of these proteins calculated from the pulse-chase data varied from 2.0 to 23 h. Twenty-seven proteins appeared to be products of post-translational modifications. One hundred fourteen proteins appeared to be stable. The molar ratios of leucyl and methionyl residues in individual proteins and in total protein were calculated by comparing their 3H/35S ratios to that of a protein with known amino acid composition. These values varied from 1.7 to 12. The half-lives of the unstable proteins did not exhibit a correlation with protein molecular weights, isoelectric points, or leucine/methionine ratios. It may be significant, however, that 6 of the 10 most unstable proteins had low leucine/methionine ratios whereas only 11% of all proteins tested had similarly low ratios.