1. The denaturation rate of partially dried crystallizable egg albumin is greatly decreased by decreasing its water content.
2. The temperature of denaturation, defined as the temperature at which half of the protein becomes insoluble in distilled water after a definite time of heating, is a linear function of the relative humidity with which the protein is in equilibrium.
3. By applying the Arrhenius equation it is shown that the rate of heat denaturation at a given temperature is an exponential function of the relative humidity.
4. The application of the observed relations to the analysis of the mechanism of thermal death of microorganisms is suggested.
5. The water content of native and heat-denatured egg albumin is determined as a function of the relative humidity of water vapor. It is shown that the heat-denatured modification takes up approximately 80 per cent as much water at all relative humidities as does native egg albumin.