Paratuberculosis is endemic in domestic and wild ruminants worldwide. Little is known about the potential for one ruminant species to act as a vector of transmission of infection to another species. Recently, paratuberculosis has been diagnosed in a population of captive bison in the western United States. Although bison develop signs of unthriftiness and suffer from severe weight loss, other clinical signs are lacking. In addition, it is difficult to culture M. paratuberculosis from feces of infected bison for definitive diagnosis. We designed the following study to compare host immune responses and pathologic changes in beef calves and bison calves after challenge with either a cattle or bison strain of M. paratuberculosis. In the first study, 6 bison and 6 beef calves were orally inoculated over a 2-week period with a cattle isolate of M. paratuberculosis. In a second study, 6 bison and 6 beef calves were similarly inoculated with a bison strain of M. paratuberculosis. Throughout each of the studies, blood and fecal samples were taken monthly for a 6-month infection period. Tissue samples were obtained at necropsy for culture and histopathologic analyses. Results from this study demonstrated that bison calves were more susceptible to tissue colonization than beef calves, regardless of bacterial strain. Although lesions were minimal they were most apparent in the jejunum and distal ileum. Interferon-gamma responses were noted in some calves by one month post-inoculation and were sustained longer in beef calves after challenge with the bison isolate. Antibody was not detected in either beef or bison calves during the 6-month infection period. These results indicate that the host response to strains of M. paratuberculosis may differ between ruminant species.