Ginseng radix (Panax ginseng C.A. Meyer) is a popular herbal medicine in Oriental countries. We investigated the effect of long-term oral administration of ginseng extract on the antigen-specific antibody response. Male BALB/c mice were treated orally for 30 consecutive days with 2 g/kg of a 50% ethanol extract of ginseng root. Mice treated with ginseng and immunized with ovalbumin (OVA), resulting in an eight-fold increase in titers of anti-OVA immunoglobulin (Ig)G in the serum compared to the group receiving OVA immunization without ginseng treatment; the level of IgG was also significantly elevated in the mice treated with ginseng and immunized with OVA. Mice treated with ginseng without OVA immunization exhibited significantly reduced IgG and IgA production by spleen cells. However, IgG production was not affected in mice treated with ginseng and OVA immunization in spleen cells. Interleukin (IL)-2, interferon (IFN)-gamma and IL-4 secretion by spleen cells from either ginseng-treated mice or OVA-immunized mice were down-regulated compared to that in the control group; while the production of IL-10 was unchanged. The percentage of CD8+ cells was significantly reduced in spleen cells from ginseng-treated, OVA-immunized mice. Thus, long-term oral administration of ginseng extract appears to potentiate humoral immune response but suppress spleen cell functions.