Milk can be contaminated with Mycobacterium paratuberculosis (MAP) by faecal contamination or direct shedding, also by asymptomatic animals. Different studies indicate that MAP would be frequently present in cow's milk at the farm and in industrial collection tanks. Because most milk and dairy products are heat treated before consumption, the effectiveness of milk pasteurization for eliminating viable MAP is crucial. Recent research has demonstrated survival of MAP under pasteurization conditions. This fact cannot simply be explained by the thermal death kinetic data obtained in artificial contamination experiments. Two hypotheses for the survival of MAP to industrial pasteurization are proposed. It is generally known that MAP cells have the tendency to clump, each clump being counted by the culturing method as 1 cell. Survival of MAP could be explained on the basis of the kinetic data with clumps of >10⁴ individual cells. Our recent research showed, however, that fecal MAP clumps from naturally infected cows are not containing more than 10 individual MAP cells making the hypothesis of survival of MAP cells in very big clumps unlikely. More recently, a not yet elucidated heat activation mechanism has been suggested as the cause of survival during pasteurization. In our hands, heating of milk samples from MAP positive dairy cows in a pilot-plant pasteurizer (10-30 min, 60-100°C) showed a strong indication that a heat activation of MAP growth takes place. Considerably more positive cultures were obtained early during culturing at the higher heating temperatures (80-100°C). With an additional chemical lysis of the milk before pasteurization there was a tendency towards more suspected positive cultures, even at lower pasteurization temperatures (60-70°C). These findings suggest the involvement of another trigger in the activation process.