Intracellular protein concentration is an essential cell characteristic which manifests itself through the refractive index. The latter can be measured from two or more mutually defocused brightfield images analyzed using the TIE (transport-of-intensity equation). In practice, however, TIE does not always achieve quantitatively accurate results on biological cells. Therefore, we have developed a calibration procedure that involves successive imaging of cells in solutions containing different amounts of added protein. This allows one to directly relate the output of TIE (T) to intracellular protein concentration C (g/l). The resultant relationship has a simple form: C ≈ 1.0(T/V), where V is the cell volume (m3) and 1.0 is an empirical coefficient. We used calibrated TIE imaging to characterize the regulatory volume increase (RVI) in adherent HeLa cells placed in a hyperosmotic solution. We found that while no RVI occurs over the first 30-60 min, the protein concentration fully recovers after 20 h. Furthermore, interpretation of such long experiments may depend on whether protein concentration varies significantly throughout the cell cycle. Our data on HeLa, MDCK and DU145 cells indicate that it remains relatively stable.