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Changing immune parameters during pregnancy have previously been reported in humans and cattle, and have been suggested to contribute to increased susceptibility to infections. However, data regarding immune parameters during pregnancy in sows are rare. In this study, we investigated the peripartal immune status of sows using phenotypical (FACS analysis) as well as functional (proliferation assays, cytokine analysis) parameters of peripheral blood mononuclear cells (PBMCs) in pregnant sows. In previous studies, we reported a modulation of the immune system after feed supplementation of the probiotic Bacillus cereus var. toyoi in piglets [Schierack, P., Wieler, L.H., Taras, D., Herwig, V., Tachu, B., Hlinak, A., Schmidt, M.F., Scharek, L., 2007. Bacillus cereus var. toyoi enhanced systemic immune response in piglets. Vet. Immunol. Immunopathol. 118, 1-11]. Here, we extended these previous studies to include investigations of possible probiotic effects on the peripartal immune status of sows and their reproductivity. We show that immune parameters of sows change during pregnancy, the proliferative response of PBMCs to several bacterial antigens in control animals decreased from days 90 to 30 ante partum. Relative numbers (%) of CD3+CD8+, CD4+, cytotoxic T, CD14+ and CD21+ cells were reduced compared to non-pregnant sows. In contrast, the proliferative response of PBMCs of probiotic-treated sows increased during pregnancy. Bacterial antigens primarily stimulated the proliferation of naïve CD21+ cells and the relative CD21+ cell numbers were elevated in the probiotic group in the absence of effects on other immune cell populations. The clinical and microbial status of both control and probiotic sows was similar, excluding pre-existing health problems or infections as responsible for the immunological changes, and feed supplementation also had no significant effects on reproductivity. The results suggest that the probiotic B. cereus var. toyoi can alter the proliferative response of lymphocytes and affects the immune cell population ratios of pregnant sows. How and to what extent this may affect health and reproductivity should be the focus of further studies.