To study the effects of lactic acid bacteria on the mucosal defence against dietary protein antigens, we compared the mucosal IgA responses to beta-lactoglobulin (beta-LG) of two groups of mice fed a whey protein diet with and without a culture condensate of Bifidobacterium longum. Both total IgA and anti-beta-LG IgA levels in tissue extracts of the small intestinal wall were significantly higher in mice fed the B. longum diet for 2 weeks than in control ones. Peyer's patch (PP) cells from B. longum-fed mice had a much larger increase in in vitro IgA production than ones from control mice. Furthermore, the in vitro IgA response to beta-LG was detected only when PP cells from B. longum-fed mice were assayed. These results suggest that orally ingested lactic acid bacteria may protect a host from invasion of the intestinal mucosa by dietary antigens that have escaped enzymatic digestion in the intestine.