Experiments were conducted to investigate reports that deer are more susceptible than sheep, cattle or goats to infections with M. paratuberculosis and M. avium (Nyange, 1990). Groups of 4 red deer calves (Cervus elaphus) were infected orally with either IS901 M. avium or M. paratuberculosis. Two calves served as non-infected controls. Regular examinations were made of blood cell-mediated immune (CMI) reactions, antibody responses and fecal excretion of mycobacteria throughout the 41 week term of the experiment. Clinical illness was observed only in deer inoculated with M. avium and fecal excretion of mycobacteria was demonstrated between weeks 3 and 25 post-inoculation. In contrast, fecal excretion of M. paratuberculosis was detected much less frequently between weeks 13 and 21 pi. Differences in immune responses to the two infections were obvious. Essentially, the group of M. paratuberculosis-infected deer exhibited low antibody and transient CMI reactivity, whereas both responses were evident early in M. avium-infected deer and were sustained throughout the 41-week period of the experiment.