Campylobacter jejuni remains the leading cause of bacterial enteritis in the UK, with over 40,000 cases reported in England and Wales in 2003. The handling and/or consumption of contaminated poultry meat is believed to be a risk factor. The avian intestine is considered the natural environment for the organism, and up to 90% of poultry flocks in the UK are colonised asymptomatically. Thus, reduction or elimination of the organism in poultry in order to reduce the number of human cases is an objective of DEFRA and FSA. A number of intervention strategies have been proposed, including increased bio-security, vaccination and competitive exclusion. Increased bio-security alone has so far failed to consistently reduce the extent of colonisation, so a multi-pronged approach would seem to offer the most promise. Commercial birds tend to exhibit a lag-phase of infection, wherein they usually remain uncolonised during the first 2-3 weeks of life. Thereafter the organism spreads rapidly through the flock. The lag-phase represents a window-of-opportunity to apply intervention strategies. Moreover, understanding the under-lying mechanisms could help in the development of successful strategies. One likely contributory factor to the lag-phase, maternally-derived immunity, will be investigated in this project. Previous DEFRA-funded work has indicated that the susceptibility of commercial birds to colonisation is inversely proportional to the levels of anti-C. jejuni antibodies present in the chicks. These antibodies are vertically transmitted from the breeder hens suggesting these birds are, or have been colonised by the organism. Their levels decline to background during the first 3-4 weeks of life. The role of maternal immunity will be studied by looking at the susceptibilities to colonisation of chicks derived from SPF and commercial flocks and from experimental hens colonised and uncolonised by C. jejuni. The results will be used to help in the development of intervention strategies, including possibly extending the lag-phase to provide a larger window-of opportunity to apply other targeted intervention approaches.