The effects of orally administered decoction of Juzen-Taiho-To (JTT; Si-Quan-Da-Bu-Tang in Chinese) on cytokine production in hepatic lymphocytes were studied in mice. JTT was found to increase interferon gamma (IFN-gamma), as well as interleukin-4 (IL-4), IL-5 and IL-6 secretion from stimulated hepatic lymphocytes, whereas IL-2 secretion was reduced. The number of IFN-gamma- and IL-4-spot forming cells (SFC) were not changed by administration of JTT. These results suggest that modulation of cytokine secretion by JTT might not be due to changes in the number of cytokine secreting cells within liver lymphocytes. CD4/CD8 ratio and alphabeta/gammadelta T cell receptor (TCR) ratio in hepatic lymphocytes were not changed. However, flow cytometric analysis revealed that the population of CD3 positive intermediate cells in NK positive cells (NKT cells) was increased after oral administration of JTT. The population of CD3int IL-2Rbeta+ cells was also increased. The induction of NKT cells by JTT was reduced by injection of 2-chloroadenosine. JTT enhanced transcription of IL-12 mRNA in liver. From these results, it may be concluded that a rise in NKT cell population contributes, at least partially, to the modulating effect of JTT on cytokine production in liver lymphocytes, and macrophages. The production of IL-12 in liver may also contribute to this NKT induction.