Soy sauce (Shoyu) is a traditional Japanese fermented seasoning and is available worldwide. We investigated the effect of Shoyu polysaccharides (SPS) prepared from soy sauce on the intestinal immune system of mice. SPS enhanced the production of immunoglobulin A (IgA) from Peyer's patch cells in vitro, and its oral administration to 7-week-old male BALB/c mice for 2 weeks at a dose of 1.5 mg per day significantly (p<0.01) increased the concentration of IgA in the intestine as compared to control mice. Furthermore, experiments on the intestinal transport of SPS in vitro using the human intestinal epithelial cell line Caco-2 confirmed the permeation of uronic acid to be time-dependent. In conclusion, SPS of soy sauce enhanced the production of IgA in vitro and in vivo, and the digested SPS might cross the enterocytic monolayer. Thus, soy sauce is a potentially promising food for enhancing host defenses.