Antibodies and cytokines in serum were detected in male ICR mice treated with the aqueous extract of Epimedii Herba (AEEH) at doses of 40, 120 and 360 mg/kg orally for 2 weeks. Effects of AEEH on antibody forming responses were assessed by enzyme linked immunosorbent assay (ELISA) of immunoglobulin (Ig) levels in serum collected 7 days after priming with ovalbumin (OVA) in complete Freund's adjuvant (CFA) or immediately without priming at week 2. The relative spleen weight was significantly increased by AEEH, compared with controls, especially at a dose of 120 mg/kg of it after priming with OVA and 40 mg/kg without priming, respectively. However, body weight gain was slightly decreased in AEEH-fed mice. The enhancement of total serum IgG and IgG1 levels in unprimed mice was statistically significant in mice fed 40 mg/kg AEEH. Total serum IgG2a levels and Il-4 secretion were also statistically augmented by all groups of AEEH treatment. A tendency to marked increase of total serum IgM level and IFN-gamma secretion was also observed in mice fed 40 and 120 mg/kg AEEH but not those fed 360 mg/kg AEEH. When mice were immunized with OVA, furthermore, a marked stimulation of antibody formation and cytokines secretion was observed in all groups of AEEH-fed mice compared with controls. These findings indicate that AEEH at therapeutic concentrations enhances the production of antibodies and cytokines in mice, and the enhancing effects are more marked when the mice were immunized with OVA. Thus, these results suggest that AEEH is effective on Th cell functions, and protective effects on host against immune diseases.