In order to assess bacterial thermosusceptibility and establish the efficacy of pasteurization, it is preferable to use commercial pasteurizing units because the heating and cooling differentials generated in laboratory heat treatment may not simulate commercial conditions. Seventeen batches of raw milk were loaded with 10²-10⁵ CFU/ml of Mycobacterium paratuberculosis and pasteurized in a small scale commercial pasteurizing unit at temperatures ranging from 72-90°C for 15-35 seconds. M. paratuberculosis was not isolated from 96% (275/286) of pasteurized milk samples, representing 10^-4 reductions in mycobacterial concentrations as radiometric culture could detect about one colony forming unit per ml. Viable mycobacteria were not recovered when raw whole milk was loaded with less than 10⁴ mycobacteria per ml. Mycobacteria were not cultured in any of 5 batches of milk pasteurized at 72-73°C for 25-35 seconds, which are the minimum conditions applied when this machine is used commercially to correct for laminar flow in the holding tube. Adequate holding time appeared to be more effective in killing M. paratuberculosis than higher temperatures in the small number of batches treated. M. paratuberculosis was cultured from 5 of the 17 batches of milk, with less than 30 colony forming units of M. paratuberculosis surviving per ml. These included 4 of 8 batches heat treated at 72-73°C for 15 seconds and one of 4 batches treated at 82-90°C for 15 seconds. Survival of M. paratuberculosis in experimentally inoculated batches of milk in the small-scale commercial unit cannot be directly extrapolated to commercial pasteurization of naturally infected milk in dairy factories because of artificially high mycobacterial loads used in these experiments, possible differences between the thermosusceptibility of laboratory cultured mycobacteria, and features of the small-scale unit. Pasteurization in a small-scale unit used in these experiments was apparently more efficient at killing mycobacteria than laboratory heat treatment systems.