Probiotics, microorganisms that have a favorable influence on physiological and pathological processes of the host by their effect on the intestinal flora, may play a role in improving human health. One of the putative effects is the modulation of immune function. The present study was carried out to evaluate the immune modulatory potential of four strains of Lactobacillus acidophilus in swiss albino mice. Immune response was assessed by various methods as the development of antibody titers, the delayed type hypersensitivity test, the nitroblue tetrazolium reduction test, inducible nitric oxide synthase test and the phagocytic activity test. The results showed that immunomodulation by L. acidophilus is strain specific and it is asserted that orally supplemented L. acidophilus could exert an indirect effect on T-lymphocyte activity through stimulation of other cell types, such as phagocytes.