Recent evidence has linked Crohn's disease, an inflammatory enteritis, in humans to infection with M. paratuberculosis. Although not characterized as a zoonotic agent, M. paratuberculosis has been identified in intestinal biopsy tissue from patients with Crohn's One possible source of human exposure to M. paratuberculosis is the consumption of dairy products, since presence of M. paratuberculosis DNA has been documented in cow's milk obtained from retail markets in Great Britain. Evaluation of optimal time/temperature combinations for effective heat inactivation of M. paratuberculosis was determined using either a holder-tube method or a lab-scale industrial pasteurization method. Raw milk was obtained from paratuberculosis - free cows was dispensed into 13 x 100 mm snap-cap polystyrene tubes and placed in a shaking water bath at either 65, 72, 74, or 76°C, with one tube serving as a temperature control. Milk was inoculated with 10⁸ CFU M. paratuberculosis /ml (strains 19698, Ben and Kay) and aliquots removed at each time point for serial dilution and culture on HEYM. Results indicate that the most effective reduction in viable bacterial numbers was achieved at 72°C, but mean time for optimal killing superseded industrial recommendations of 15 seconds at that temperature. Studies with the lab-scale pasteurizer were conducted in a similar manner. Raw milk (1-2 liters) inoculated with M. paratuberculosis (10⁴, 10⁶, and 10⁸ CFU/ml; strains 19698, Ben, and Kay) was poured into the holding vessel and circulated for 15", 72C. Samples were obtained from the output tube at the beginning, middle, and end of the pasteurization run for culture. Results from all experiments conducted with the lab-scale pasteurizer demonstrated that treatment of raw milk inoculated with live M. paratuberculosis at 72C, 15", effectively killed all the bacteria. Comparison of these two models for testing heat inactivation of M. paratuberculosis indicate that the lab-scale pasteurizer more closely simulates industry conditions and that results from those experiments should be given more careful consideration.