An IgG1 ELISA (Paralisa™) test has been developed to diagnose infection and disease caused by Mycobacterium avium subsp. paratuberculosis (Map) in deer. This technique was used to estimate the prevalence of Johne's disease in juvenile and adult deer on three large New Zealand deer farms, known to harbour infection with Map. Using an empirical cut point of 50 ELISA units of reactivity to Map antigens, the incidence of reactors varied between 6 to 40%. Lesion rates among reactor animals varied between 30 to 87% in adult animals. A proportion of seropositive animals did not show any clinical symptoms or gross pathology but had histopathological lesions or were culture (+) on examination at slaughter. This suggests that IgG1 ELISA is capable of detecting subclinically infected animals. Sequential Paralisa tests carried out on young deer showed a strong correlation between positive reactivity at eight months of age, and a poor prognosis for survival and production performance subsequently. Animals remaining negative on sequential tests had satisfactory production levels. A small group of animals, which converted from negative to positive reactivity between 8 - 12 months of age showed satisfactory growth rates. Longitudinal studies carried out over a three year period suggest that this technology can be used to control Johne's disease on deer farms. Details of the methodology will be discussed. Performance parameters (sensitivity/ specificity) have now been evaluated using a large cohort of naturally and experimentally infected deer from throughout New Zealand. The performance of the Paralisa to date suggests that it may have a role to identify and cull of Map infected juvenile and adult deer.