The irreversible thermal inactivation of Bacillus licheniformis alpha-amylase was studied. A two-step behaviour in the irreversible denaturation process was found. Our experimental results are consistent only with the two-step model and rule out the two-isoenzyme one. They suggest that the deactivation mechanism involves the existence of a temperature-dependent intermediate form. Therefore the enzyme could exist in a great number of active conformational states. We have shown that Ca2+ is necessary for the structural integrity of alpha-amylase. Indeed, dialysis against chelating agents leads to a reversible enzyme inactivation, though molecular sieving has no effect. Further, the key role of Ca2+ in the alpha-amylase thermostability is reported. The stabilizing effect of Ca2+ is reflected by the decrease of the denaturation constants of both the native and the intermediate forms. Below 75 degrees C, in the presence of 5 mM-CaCl2, alpha-amylase is completely thermostable. Neither other metal ions nor substrate have a positive effect on enzyme thermostability. The effect of temperature on the native enzyme and on one intermediate form was studied. Both forms exhibit the same optimum temperature. Identical activation parameters for the hydrolytic reaction catalysed by these two forms were found.