The Peyer's patches contain a large number of precursor cells committed to the production of immunoglobulin A (IgA) and play an important role in IgA response in the mucosal immune system. We investigated the induction of IgA producing cells in Peyer's patches by plaque forming cell assay after oral administration of a traditional Chinese herbal medicine, Xiao-chai-hu-tang (Japanese name: Shosaiko-to). The number of total IgA producing cells in the Peyer's patches detected by the protein-A plaque assay was increased about two-fold by Shosaiko-to administration and the numbers of both anti-SRBC and anti-HRBC IgA producing cells were also increased by such a treatment. On the other hand, when SRBC alone were administered orally, only the number of anti-SRBC IgA producing cells was increased. Further, we examined T-cell subpopulations in the gut-associated lymphoid tissues after oral administration of Shosaiko-to by flowcytometry. Marked alternations in T cell subpopulations were not detected in the Peyer's patches, though TcR gamma delta+T-cells in the intraepithelial lymphocytes and Thy1.2-TcR alpha beta+T cells in the mesenteric lymph nodes were slight increased. These results showed that orally administered Shosaiko-to acts as a polyclonal B-cell activator which induces IgA production in the mucosal immune system.