The objective of this dissertation was to examine T cell and macrophage responses during natural and experimental disease caused by Mycobacterium avium subspecies paratuberculosis. In the initial experiment, euthymic BALB/c and athymic nude BALB/c mice were infected with M a. paratuberculosis . Five months post-inoculation, euthymic mice differed from athymic mice in that (1) their hepatic granulomas were smaller, contained fewer bacteria, and produced more inducible nitric oxide synthase (iNOS), and (2) their hepatic macrophages contained fewer bacteria, a higher percentage of degraded bacteria, and increased numbers of primary lysosomes. The study showed that macrophage activation was markedly decreased in the T-cell deficient athymic mice. In the second study, calves from M. a. paratuberculosis infected cows and calves from non infected cows were inoculated with M a. paratuberculosis or saline. The calves were monitored for evidence of exposure to M. a. paratuberculosis for 7 months by IFN-gamma assays, skin testing, serology, and fecal culture. Calves from dams subclinically infected with M. a. paratuberculosis had significantly higher IFN-gamma responses to mycobacterial antigens, prior to inoculation on day 1, and cumulatively throughout the experiment than calves from non-infected dams. Two calves from infected darns had positive skin test responses and M. a. paratuberculosis isolated from tissues at the end of the experiment. This study showed that maternal infection resulted in an increase in IFN-gamma production to mycobacterial antigens by their calves. The final study examined the immunohistochemical and ultrastructural pathology of the ileum in bovine Johne's disease and quantified CD3 lymphocytes, CD4 lymphocytes, CD8 lymphocytes, gammadelta lymphocytes, CD79 lymphocytes, CD68 cells, iNOS and mycobacterial immunoreactivity. Cows with Johne's disease had increased immunoreactivity in epithelial areas for CD3 and gammadelta antigen, while in the lamina propria there was increased immunoreactivity for CD68 and mycobacterial antigen as compared to control cows. Inducible nitric oxide synthase immunoreactivity occurred only in the cows with Johne's disease and was limited to macrophages and multinucleate giant cells located in the submucosa and serosa. Ultrastructural analysis of M. a. paratuberculosis infected macrophages revealed a hypertrophied vacuolar transport system and numerous structurally intact bacilli that were located in heterogeneous bacteriophorous vacuoles.