Increasing numbers of functional foods and pharmaceutical preparations are being promoted with health claims based on the potential probiotic characteristics of lactic acid bacteria and on their capacity for stimulating the host immune system. However, the specific immune effects of oral administration of these microbes still remains undefined. In this study, we tested the hypothesis that production of immunologic mediators by leukocytes in mice is affected by orally administered lactic acid bacteria. The specific objectives of this study were to evaluate the effects of exposure to eight different lactic acid bacteria in mice on ex vivo cytokine and nitric oxide production in leukocyte cultures. Mice were gavaged with 1 X 10(9) viable bacteria and peritoneal, Peyer's patch and splenic leukocytes were isolated 8 h later. These were cultured for 2 or 5 days in the presence or absence of mitogens and then interleukin (IL)-6, IL-12, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, and nitric oxide production was measured. The results revealed that Lactobacillus acidophilus and L. casei potentiated IL-6 and IL-12 production by peritoneal cells whereas L. acidophilus upregulated IFN-gamma and nitric oxide. In contrast, L. helveticus, L. gasseri, L. reuteri, and Bifidobacterium attenuated the production of IL-6, IFN-gamma, and nitric oxide by peritoneal cells. TNF-alpha was not detectable in peritoneal cultures. None of the bacteria altered ex vivo production of cytokines or nitric oxide by Peyer's patch or spleen cell cultures. Taken together, the results suggest that prior oral exposure to lactic acid bacteria could differentially potentiate or attenuate subsequent cytokine and nitric oxide production by peritoneal cells.