Probiotics are claimed to beneficially affect the immune system and their involvement in allergy prevention is being investigated extensively. However, the efficacy of probiotics in allergy prevention remains controversial. We investigated whether the probiotic Lactobacillus casei Shirota (LcS) could modulate the food allergic response against peanut extract (PE) in Brown Norway (BN) rats. For this purpose BN rats were sensitized to PE (0, 1 and 10mg/(ratd)) by daily oral gavage and the LcS-groups were additionally orally dosed with 1x10(9) colony forming units LcS/(ratd). LcS administration had minor effects in animals that were not sensitized. LcS increased Th1-(PE-specific IgG1), whereas the Th1/Th2 ratio based on PE-specific IgG1/PE-specific IgG2a shifted towards Th2 dominance in rats sensitized to PE in the presence of LcS as compared to rats that were sensitized to PE only. LcS stimulated PE-specific IgG2a; but for PE-specific IgE the effect was less clear; whereas there was no overall effect, two rats did not show detectable specific IgE antibodies, whereas the remainder showed significantly increased levels. LcS also resulted in increased numbers of basophilic granulocytes in blood. Furthermore, LcS increased levels of both Th1-(IFN-gamma) and Th2-(IL-4) related cytokines in PE stimulated spleen and mesenteric lymph node (MLN) cells, but predominantly IL-4 levels in the supernatants of both spleens and MLNs. Our study does not support the hypothesis that LcS down-regulates food allergic responses in a BN rat model for food allergy to peanut.