Twenty eight lambs, aged 1.0-1.5 months, were inoculated orally with approximately 4.4x10⁸ Mycobacterium paratuberculosis (M. paratuberculosis) organisms. Two weeks later 14 of these 28 animals were injected subcutaneously with 1.0 ml of a live-attenuated vaccine. Thirteen lambs were used as negative controls. Production of gamma-interferon (IFN-gamma) in blood, and antibody levels in serum were sequentially monitored in each sheep. The infection status of each sheep was determined by histology and an IS900-based PCR test on samples of ileum and ileocaecal lymph node at the time of necropsy. Between the 27th and 53rd week postinfection, 7 infected/unvaccinated (unvaccinated) and 2 infected/vaccinated (vaccinated) animals developed clinical signs consistent with paratuberculosis. In vaccinated sheep, an initial rise in both the IFN-gamma response and antibody titre was observed in the 5th week postinfection, while in unvaccinated sheep, an increase in IFN-gamma response was not detected until 18 weeks postinfection, and no increase in antibody level was detected until 27 weeks. Between the 5th and 53rd week postinfection, both the IFN-gamma and antibody concentrations were greater in the vaccinated group than in the unvaccinated group. The granulomatous inflammation in the jejunal and ileal mucosa was less severe in the group of vaccinated sheep than in the group of unvaccinated sheep. Acid fast organisms were detected only in the latter group. The PCR assay on ileal samples gave positive reactions in 2 vaccinated and 8 unvaccinated sheep. These data indicate that vaccination of lambs with live-attenuated vaccine 2 weeks after oral inoculation with M. paratuberculosis stimulated the host response against the organism and led to a reduced mycobacterial burden. Furthermore, the mean IFN-gamma production was higher in 7 clinically normal unvaccinated sheep than in 7 unvaccinated sheep with clinical paratuberculosis. This suggests a positive relationship between the magnitude of peripheral cell-mediated response and ability to control the infection.