An ethanol extract of Semen Cuscutae (EESC) was evaluated for its adjuvant potentials on the cellular and humoral immune responses of ICR mice against ovalbumin (OVA). ICR mice were immunized subcutaneously with OVA 100 microg alone or with OVA 100 microg dissolved in saline containing aluminum hydroxide gel (Alum) (200 microg), Quil A (10 or 50 microg) or EESC (100, 200 or 400 microg) on Days 1 and 15. Two weeks later (Day 28), concanavalin A (Con A)-, pokeweed (PWM)- and OVA-stimulated splenocyte proliferation and OVA-specific antibody in serum were investigated. EESC significantly enhanced the Con A-, PWM-, and OVA-induced splenocyte proliferation in OVA-immunized mice at a dose of 200 microg (P<0.05 or P<0.025). OVA-specific IgG, IgG1 and IgG2b antibody levels in serum were significantly enhanced by EESC compared with OVA control group (P<0.025). Moreover, enhancing effects of EESC on these OVA-specific antibody responses to OVA in mice were more significant than those of Alum and Quil A (P<0.025). In conclusion, the results suggest that EESC is effective on Th1 and Th2 cell functions, and could be safely used as adjuvant.