Two longitudinal experiments involving Friesian cattle, Merino sheep and Angora goats (5 animals per group) challenged with either bovine or ovine strains of Mycobacterium avium subspecies paratuberculosis have been conducted over a period of 48 and 31 months, respectively. The experiments are still in progress. The age of the cattle, sheep and goats challenged with the bovine Johne's disease (BJD) strain was 6 weeks, 6 months and 5 months, respectively whereas those challenged with the OJD strain were 14 weeks, 10 months and 10 months. Four doses of BJD or OJD cultured bacteria (1x10¹⁰ - 2x10¹⁰ per ml) and either BJD or OJD mucosal tissue (15 - 20 g) were given orally at weekly intervals for 4 weeks. Control animals for each of the species were dosed with broth diluent. Blood samples for the bovine gamma interferon test (BOVIGAM^TM, CSL Limited) and the Johne's absorbed EIA (PARACHECK^TM, CSL Limited) and faecal samples for conventional culture and radiometric culture (BACTEC^TM, Becton Dickinson) were taken pre-challenge and monthly post-challenge. There were differences between the species in their bacteriological and immunological responses to infection. Goats appear to be the most susceptible to BJD infection than cattle or sheep in terms of clinical disease, seroconversion and persistent bacterial shedding. Goats also appear to be more susceptible to BJD than OJD. The BJD affected goats developed persistently elevated interferon-g and antibody responses around the time that shedding commenced with interferon-g declining to baseline levels in the terminal stages of the disease. The presence of high background interferon-g levels in the control cattle, obtained from different farms, in both the BJD and OJD experiments made it difficult to interpret the results of the test in the challenged cattle groups. However, the results do indicate that for sheep the interferon-g test may be potentially useful for determining if a flock has been exposed to OJD.