Thermal resistance of intracellular and freely suspended Listeria monocytogenes that was associated with a milkborne outbreak of listeriosis was studied by using the sealed tube and slug flow heat exchanger methods. Test temperatures for the former method were 57.8, 62.8, 66.1, and 68.9 degrees C (136, 145, 151, and 156 degrees F, respectively); whereas those for the latter method were 66.1, 68.9, 71.7, and 74.4 degrees C (151, 156, 161, and 166 degrees F, respectively). The heating menstruum was sterile, whole milk. The intracellular inoculum was generated from an in vitro phagocytosis reaction by using endotoxin-induced bovine milk phagocytes. The phagocyte population consisted of 88% neutrophils, 8% macrophages, and 4% lymphocytes. Neutrophils harbored the majority of intracellular L. monocytogenes. The mean level of infectivity in the phagocyte population was 43%, and there were 26.1 +/- 19.3 bacteria per cell (10(4) viable cells per ml of test milk). Initial bacterial counts for the freely suspended and intracellular experiments (the latter was based on a sonically disrupted sample) were 10(6) L. monocytogenes cells per ml. Heat-stressed bacteria were recovered by direct plating in parallel with recovery from an enrichment broth; both methods gave comparable results. The predicted D62.8 degrees C (145 degrees F) value for intracellular sealed tube studies was 53.8 s (ZD = 5.6 degrees C [10.0 degrees F]), indicating a safe 33.4 D margin of inactivation for vat pasteurization (62.8 degrees C for 30 min).(ABSTRACT TRUNCATED AT 250 WORDS)