A parametric correlation (p less than 0.01) has been found between the in vitro thermal stability and in vivo turnover rates of nine intracellular proteins. These results are discussed in terms of a "thermodynamic" model for turnover control, in which the rate of intracellular protein degradation is controlled by intramolecular conformation equilibria. A peculiar exception is provided by glyceraldehyde-3-phosphate dehydrogenase which is stable in vivo, but not in vitro.