Aim: To investigate the cell viability of Bifidobacterium longum 51A in fermented milks and to study its immunostimulating and protective capacity against Salmonella enterica ssp.similar to enterica serovar Typhimurium infection in mice. Methods and Results: Bifidobacterium longum 51A was added to milk fermented with different yoghurt starter cultures, before or after fermentation, and viability was monitored during storage (5 degrees C, 28 days). Resistance to simulated gastric acid digestion was assessed. Fermented milks were orally administered to mice for 10 days followed by oral infection with Salmonella Typhimurium. The number of IgA+ cells in the small and large intestine was determined before infection. Survival to infection was monitored for 20 days. Bifidobacterium longum 51A lost viability during storage, but the product containing it was effective for the induction of IgA+ cells proliferation in the gut and for the protection of mice against Salm.similar to Typhimurium infection. Conclusions: Cell viability of Bif.similar to longum 51A in fermented milks along storage did not condition the capacity of the strain to enhance the number of IgA+ cells in the gut and to protect mice against Salmonella infection. Significance and Impact of the Study: The uncoupling of cell viability and functionality demonstrated that, in certain cases, nonviable cells can also exert positive effects.