Protein obtained from several strains of Escherichia coli grown in the presence of [3,3'-14C]cystine contained the radiolabel in nearly all the other amino acids, suggesting catabolism of cysteine to pyruvic acid. Utilization in amino acid synthesis of the pyruvate thus generated can be blocked by growing the bacteria in a medium specifically enriched with most of the naturally occurring amino acids. Cysteine that is incorporated intact is diluted by de novo synthesis at low cystine concentrations; also, it was found that E. coli can use the sulfur of methionine for cysteine biosynthesis. Both of these latter two processes can be prevented by supplying an excess of exogenous cystine. This regimen leads to protein that is highly specifically labeled in the cysteine residues, with a minor amount (20-25%) of the label also appearing in alanine residues. Although this strategy was developed expressly for cysteine, it may be useful for incorporating other labeled amino acids that are also readily catabolized.