The inactivation of the endogenous milk enzymes alpha-L-fucosidase (EC 3.2.1.51), phosphohexoseisomerase (EC 5.3.1.9), phosphodiesterase I (EC 3.1.4.1) and alpha-mannosidase (EC 3.2.1.24) during heating of milk follows a first-order reaction. Within times of up to 800 s, alpha-fucosidase is inactivated in the temperature range between 52 degrees C and 62 degrees C (activation energy, E(A) = 378 +/- 11 kJ/mol) and phosphohexoseisomerase between 56 degrees C and 66 degrees C (E(A) = 362 +/- 8 kJ/mol), which allows a characterisation of the thermisation process. Phosphodiesterase I is inactivated during pasteurisation (up to 1600 s at 65 degrees C to 78 degrees C; E(A) = 493 +/- 9 kJ/mol). alpha-Mannosidase activity is suitable to describe heating within the lower range of high-temperature pasteurisation (up to 1000 s at 76 degrees C and 86 degrees C; E(A) = 553 +/- 11 kJ/mol). The mean values of enzyme activities in raw milks of individual cows varied between 16 and 50%. Compared to this, only 1.8 to 4.8% variation could be detected in bulk milk collecting tanks. No enzyme activity could be measured in 21 commercial UHT milk samples, indicating that no reactivation occurs. In 18 pasteurised milks, up to 15% residual activity of gamma-glutamyltransferase and up to 30% residual activity of phosphodiesterase were detected, indicating a heat treatment at the upper limits.