Sho-saiko-to (SST), a Chinese/Japanese traditional herbal medicine, has been widely used to treat chronic hepatitis in Japan, and the immunomodulatory properties of SST are likely to mediate its beneficial effect. In the present study, we examined the effects of SST and its various ingredients on the count and proliferation of T-cell subsets in cultured splenocytes and hepatic mononuclear cells. SST, wogonin-7-O-glucuronoside (a major SST ingredient), and wogonin (an intestinal metabolite of wogonin-7-O-glucuronoside) increased CD4/CD8 ratio via a decrease of CD8+ T-cell counts with no effect on CD4+ T-cell counts. Flow cytometric analyses of viability, proliferation, and cell cycle revealed that wogonin suppressed CD8+ T-cell proliferation without inducing cell death. SST and wogonin administered to mice increased the CD4/CD8 ratio in hepatic mononuclear cells but not in splenocytes. These findings suggest that SST may modulate the CD4/CD8 ratio via the selective inhibition of CD8+ T-cell proliferation by the SST ingredient wogonin-7-O-glucuronoside or its metabolite wogonin.