The relative percentage and absolute numbers of peripheral blood mononuclear cells (PBMC) bearing CD4, CD8, or gamma T lymphocyte receptors were investigated in calves with experimentally induced paratuberculosis. Five female Holstein calves were orally challenged with 10⁸ M. paratuberculosis at the age of 4 weeks. Four breed, age, and sex-matched calves served as controls. Every 4 weeks to 20 months post-infection, PBMCs were isolated, labeled with commercial monoclonal antibodies against the CD3, CD4, CD8, or gamma T cell receptors, stained using a second fluorescein-labeled antibody, and analyzed by flow cytometry. For each calf at each sampling, a total white blood cell count and differential were performed in order to calculate absolute numbers of each T cell subset from the flow cytometry data. The 5 calves orally challenged with M. paratuberculosis were confirmed infected by culture or surgically obtained ileum and regional lymph nodes. A linear increase in number of CD4+ T cells from 627/mcl to 2828/mcl occurred in both control and infected calves from ages 4 to 14 months. CD8+ T cell numbers varied considerably between sample dates but were relatively constant over the study period (576 ± 100/mcl). The number of gamma T cells in control calves increased from 1420 ± 395/mcl to 3020 ± 747/mcl from month 4 to month 10 of the study, and then declined to levels comparable to the controls. The number of gamma T cells in the infected calves remained constant and significantly lower than in control calves (1208 ± 386/mcl). Peripheral blood neutrophil counts were significantly lower in the M. paratuberculosis-infected calves over approximately the same time period. The M. paratuberculosis infection appears to have caused a decrease in number of peripheral blood neutrophils and gamma T cells. Mechanisms for this observation are not known, but could include recruitment of the affected cells to the site of infection, direct cytotoxic effect of M. paratuberculosis products on these cell populations, or indirect effects of cytokines induced by the infection on bovine neutrophils and gamma T cells.