Aims: The effect of lactic acid bacteria on the immune system is well established under normal conditions and generally by in vivo determinations, but few data are available, in vivo, during an infectious challenge. The objective of this study was to obtain data on the putative protective role of bifidobacteria upon challenge with an intestinal pathogen. Methods and Results: The effect of oral treatment with Bifidobacterium longum Bb46 on intragastric challenge with Salmonella Typhimurium was studied. Faecal bacterial levels were determined in gnotobiotic (GN) mice and mortality, histopathology (intestines, liver), immunoglobulin levels (IgM, IgG, IgG1, IgG2a) and cytokine production (IFN-gamma, IL-10) were determined in conventional (CV) mice. Conventional mice received 0.1 ml probiotic milk (10(8) CFU) daily, 10 days before the oral pathogenic challenge (10(2) CFU). Then, probiotic treatment was continued until the end of the experiment. Probiotic treatment in germ-free mice consisted of a single dose at the beginning of the experiment. Control groups were treated with sterile skim milk and submitted to the same procedure. A higher survival (40%) was observed for probiotic-treated animals when compared with the control group (0%). This protective effect was confirmed by the histopathological and morphometric data. However, S. Typhimurium population levels in the faeces were similar among control and probiotic-treated groups. During the challenge with S. Typhimurium, a decrease in IFN-gamma and IgG2a productions was observed in probiotic-treated mice. Conclusions: The protective effect against the pathogenic challenge may be due to a reduced inflammatory response, mediated by the probiotic treatment, but not to a population antagonism. Significance and Impact of the Study: Results suggest that dietary supplementation with B. longum could provide benefits against enteric infection.