The possibility that milk from cattle with Johne's disease could be a potential vehicle of transmission of Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis) to humans has been the focus of a UK government-funded research programme at Queen's University, Belfast since 1993. The main findings of this research programme are reported and practical advice about the most appropriate methods for the isolation/detection of this organism in milk is given. The findings of several milk surveys during which optimised sensitive detection methods were employed (decontamination with 0.75% cetyl pyridinium chloride for 5 h prior to culture and a novel immunomagnetic PCR technique) have revealed that detectable levels of M. paratuberculosis are present in bulked raw cows' milk in the UK at both the farm level and at dairy processing plants prior to pasteurisation. Furthermore, results of three different experimental approaches to assess the effect of pasteurisation time/temperature conditions on the viability of M. paratuberculosis (laboratory pasteurisation studies, a national survey of commercially pasteurised milk, and processing of naturally infected milk through commercial-scale pasteurising plant) provide firm evidence that this organism is capable of surviving commercial milk pasteurisation on occasion. Hence, both raw and pasteurised cows' milk are potential vehicles of transmission of M. paratuberculosis to humans.