A study was undertaken to determine if Mycobacterium paratuberculosis survives pasteurization of milk. Twelve strains of M. paratuberculosis of bovine, ovine and caprine origin, were inoculated into aseptically-drawn cow's milk of low initial bacterial count at levels of 10⁸ cells ml-1 or 10⁴ cells ml-1 and pasteurized in the laboratory by two methods: (1) the standard holder method in which inoculated milk (5 ml) in stoppered tubes was heated to and held at 63.5°C for 30 minutes in a water-bath, exclusive of come-up time; and (2) a High Temperature-Short Time (HTST) method in which inoculated milk (250 ml) was transferred to a Franklin Plate Heat Exchanger situated in a water-bath and heated to and held at 71.7°C for 15 seconds, exclusive of come-up time (equivalent to commercial HTST pasteurization). Additional heating times of 5, 10, 15 and 20 min at 63.5°C were also included to enable the construction of a thermal death curve for the organism at this temperature. Viability after pasteurization was determined by growth on Herrold's egg yolk agar slopes containing mycobactin and in BACTEC Middlebrook 12B radiometric medium supplemented with mycobactin and egg yolk suspension. Both media were incubated at 37°C for up to 18 weeks. Any growth observed was tested for acid-fastness by the Ziehl-Nielsen stain. The thermal death curve obtained for M. paratuberculosis was concave in shape exhibiting a rapid initial death rate followed by significant "tailing" which would tend to indicate survival at low levels after pasteurization. In general growth was detected earlier by radiometry than by conventional culture on Herrold's slopes, and heat-treated M. paratuberculosis required a longer incubation period than unheated controls before growth was detected by either method which suggests the existence of sublethally-injured M. paratuberculosis following pasteurization. Preliminary results suggest that the organism may not have been completely inactivated by pasteurization at 63.5 and 71.7°C under laboratory conditions. However, the levels of M. paratuberculosis employed in this study are unlikely to be encountered in raw milk under normal circumstances and were selected to represent a "worst case" scenario.