Weaned lambs, kids and calves (5 animals per group) were experimentally infected with either cultured bovine Mycobacterium paratuberculosis or organisms in intestinal mucosal scrapings from naturally infected cows at 6 months, 5 months and 6 weeks of age, respectively. Four doses of cultured cells (1x10¹⁰ or 2x10¹⁰ per ml) or mucosa (15 to 20 g) were given orally at weekly intervals for 4 weeks. Control lambs, kids and calves (5 animals per group) were dosed with broth diluent. Blood samples for the bovine gamma interferon (IFN-gamma) test (Bovigam®, CSL Limited) and the Johne's absorbed EIA (Parachek®, CSL Limited) and faecal samples for conventional culture and radiometric culture (BACTEC®, Becton Dickinson) were taken pre-challenge and monthly post-challenge. Whilst the experiment is still in progress, these are the results up to five months post-challenge. All lambs in the mucosa dosed group and 2/5 of the organism dosed group became culture positive at 2 mo. Only 1 lamb in the mucosa dosed group remained culture positive at the 3 and 4 months sampling.. At 2 mo, 3 of the lambs in the mucosa group had elevated avian PPD IFN-gamma responses. At 3 to 5 mo all of the challenged lambs had positive avian PPD IFN-gamma responses. There were 3, 4 and 2 culture positive kids at 2, 3 and 4 mo, respectively with 2 kids shedding for at least 2 sampling periods. Avian PPD IFN-gamma responses became evident at 3 mo. Both lambs and kids challenged with mucosa generated higher IFN-gamma responses. Johnin PPD generally gave higher IFN-gamma responses than avian PPD. No IFN-gamma responses were detected in the lamb and kid controls. Three calves in the mucosa group at 3 and 4 mo were culture positive. Interestingly, challenged and control groups of calves had similarly high avian PPD IFN-gamma responses. The IFN-gamma responses in the controls may be due to the calves being sourced from an unknowingly infected farm. Of all the animals tested, only 2 kids were antibody positive in the absorbed EIA at 4 and 5 mo. The data suggests that the IFN-gamma response may be used for the early detection of Johne's disease in sheep.