Platycodin D2 (PD2), a saponin isolated from the root of Platycodon grandiflorum, was evaluated for its haemolytic activity and adjuvant potential on the cellular and humoral immune responses to ovalbumin (OVA) in ICR mice. The HD(50) values of PD2 and Quil A were 18.57+/-1.37 and 5.76+/-0.23 microg/ml against 0.5% rabbit red blood cell, respectively, implicating the less haemolytic activity for PD2 than Quil A. ICR mice were immunized subcutaneously with OVA 100 microg alone or with OVA 100 microg dissolved in saline containing Alum (200 microg), or PD2 (25, 50, 75 or 100 microg) on Day 1 and 15. Two weeks later (Day 28), concanavalin A (Con A)-, lipopolysaccharide (LPS-) and OVA-stimulated splenocyte proliferation and OVA-specific antibodies in serum were measured. PD2 significantly enhanced the Con A-, LPS-, and OVA-induced splenocyte proliferation in the OVA-immunized mice (P<0.05, P<0.01 or P<0.001). OVA-specific IgG, IgG1, IgG2a, and IgG2b antibody titers in serum were also significantly enhanced by PD2 compared with OVA control group (P<0.05, P<0.01 or P<0.001). Further, the effects of PD2 on mRNA expression of cytokines and transcription factors in Con A-stimulated mice splenocytes were evaluated by RT-PCR analysis. PD2 significantly promoted the mRNA expression of cytokines IL-2, IFN-gamma, IL-4, and IL-10 and transcription factors T-bet and GATA-3 (P<0.05, P<0.01 or P<0.001). In conclusion, the results suggest that PD2 could be safely used as adjuvant eliciting Th1 and Th2 immune responses.