Methods for producing an isotope-labeled mammalian, including a human, biomolecule, such as polypeptides and proteins, in a cell-free protein synthesis system. A biomolecule standard is produced having at least one isotope different in abundance than that of the naturally occurring isotopes in the biomolecule. Methods for quantifying biomolecules standards expressed using mammalian cell-free extracts are disclosed. Methods for producing such standards, kits, systems and reagents, relating to the use of isotope-labeled biomolecule as quantification standards in mass spectrometric and nuclear magnetic resonance analysis.