A crude ginseng extract (GS) and the purified ginsenoside R(b1) (R(b1)) were evaluated for their adjuvant effects in dairy cattle at immunisation with ovalbumin (OVA) and/or a Staphylococcus aureus bacterin used for prevention of bovine mastitis. To evaluate a suitable dose of GS as an adjuvant, 36 lactating cows were randomly divided into six groups. The cows were inoculated twice intramuscularly with a 2-week interval, with saline solution, OVA in saline, or OVA in combination with 4, 16 or 64 mg GS, or Al(OH)(3). The level of specific antibodies to OVA in serum and milk whey was measured before immunisations and 1-5 weeks after the second immunisation. The antibody response in serum was significantly higher in animals immunised with OVA and GS than in animals immunised with OVA alone. A significant increase in milk antibody titres compared with OVA only was only found 2 weeks after the second immunisation in the group immunised with OVA and 4 mg GS. In the second part of the study, 18 heifers were randomly divided into three groups and were immunised twice intramuscularly with a two week interval, with the S. aureus bacterin (control), or with the bacterin in combination with 4 mg GS or 1mg R(b1). The specific antibody response to S. aureus and the lymphocyte proliferation after stimulation with PWM, concanavalin A (Con A) or a specific S. aureus antigen was evaluated in blood samples taken before and after immunisations as specified above. Addition of R(b1) resulted both in significantly higher antibody production and lymphocyte proliferation in response to PWM, Con A and S. aureus antigens than in the control group. Addition of GS induced a significantly higher lymphocyte proliferation in response to PWM and Con A than the control, but had no additional effect on the antibody production. In conclusion, both GS and R(b1) were safe adjuvants, and R(b1) had the strongest adjuvant effects, when used for immunisation against S. aureus in dairy cattle. Field trials are warranted to test the ability of GS and R(b1) to enhance the efficacy of mastitis vaccines in protection against intramammary infections.