Thermal inactivation of a keratinase produced by Purpureocillium lilacinum LPSC #876 was kinetically investigated using several enzyme inactivation models at the temperature range of 50-65 A degrees C. Among the models studied, the Weibull distribution was the best model that describes the residual activity of P. lilacinum keratinase after heat treatment over the selected temperatures. The stabilising effect of metal ions (Ca2+ or Mg2+, 5 mmol l(-1)) or polyols (propylene glycol and glycerol, 10 % v/v) was investigated, showing that the presence of Ca2+ increases the enzyme stability significantly. Conforming to the increased Ca2+ concentration, thermal stability of the enzyme also increased, with 10 mM of Ca2+ being the concentration of metal in which the enzyme retained 100 % of its original activity after being incubated for 1 h at 55 A degrees C. The effects of temperature on Weibull equation parameters and on the characteristics of the inactivation curves were evaluated. In the absence of any additives (control), the reliable time (t (R)) of the keratinase, analogous to D value, ranged from 484.16 to 63.67 min, while in the presence of Ca2+ the t (R) values ranged from 6,221 to 414.95 min at 50-65 A degrees C. P. lilacinum keratinase is a potentially useful biocatalyst, and therefore, kinetic modelling of thermal inactivation addresses an important topic for its application in various industrial processes.