Straightforward methods for the introduction of stable isotopes into proteins with subsequent isolation and purification of the proteins will greatly aid the field of quantitative proteomics. Proteins containing amino acids with one or more of the stable isotopes of deuterium, 13C, 15N or 18O can be used as internal standards by addition at an early stage of analysis of a complex protein sample and subsequent measurement using mass spectrometry. There are two approaches for introducing a stable isotope into a protein without chemically modifying that protein, metabolic labeling using whole cells grown in culture, or cell-free labeling using a lysate of either Escherichia coli or wheat germ. Each approach has its advantages and disadvantages which will be discussed. Particular attention will be paid to the cell-free method using an E. coli lysate.