Cellular reactions to oxidative stress always include a response in the protein turnover. Therefore, cellular handling of proteins is important to observe. In this method review, radioactive labeling of proteins in vitro and in intact cells is described. The use of techniques based on the radioactive quantification of amino acids is much more selective and reliable than other nonradioactive methods for studying the protein turnover of both long- and short-lived proteins. Variations of such measurements allow one to measure protein synthesis, protein degradation, formation of insoluble proteins, and, perhaps, the turnover of individual proteins.