The immune responses of 390 BALB/c mice fed the probiotic Lactobacillus acidophilus strain NP51 ^® and infected with Mycobacterium avium subspecies paratuberculosis (MAP) were evaluated in a 6-month trial. Mice were randomized to nine treatment groups that fed either viable- or heat-killed NP51 and inoculated with either viable- or heatkilled MAP or sterile phosphate-buffered saline. Feeding the NP51 resulted in higher numbers of T lymphocytes in the spleen including the CD8 ⁺ cytotoxic T lymphocytes. In addition, feeding the NP51 lowered the number of immune suppressive T regulatory cells CD4 ⁺ CD25 ⁺ and CD8 ⁺ CD25 ^{+ cells in the spleen. Additionally, feeding the NP51 resulted in higher concentration of interferon-gamma in the supernatant of splenocytes cultured in vitro. These results suggest that feeding the NP51 to BALB/c mice might prevent the progression of MAP infection in mice.}