Objective: To evaluate the immune-stimulatory potential of extracts of Broussonetia kazinoki Siebold (BK) on specific cellular and humoral immune responses in ovalbumin (OVA)-immunized mice. Material and Methods: C57BL/6 mice were immunized intraperitoneally with OVA/alum ($100{mu}g/200{mu}g$) on days 1, 8, and 15. BK (100, 300 or 1000 mg/kg) was given to mice orally for 21 days (from day 1 to day 21). At day 22, OVA-, lipopolysaccharide (LPS)- and concanavalin A (Con A)-stimulated splenocyte proliferation and OVA-specific and total antibodies were measured in plasma. Further, the effects of BK on expression of cytokine mRNA in OVA-immunized mice splenocytes were evaluated by RT-PCR analysis. Results: BK significantly enhanced OVA-, LPS-, and Con A-induced splenocyte proliferation in OVA-immunized mice (p<0.01). BK also significantly enhanced total IgM and OVA-specific IgG1 levels in plasma compared with the OVA control group. Moreover, BK up-regulated significantly the expression of mRNA level of IL-2 and IFN-${gamma}$ in splenocytes. Conclusions: BK has immune-stimulating activity in an OVA-immunized mouse model system, enhancing the Th1 immune response. BK showed no cytotoxicity in this system, suggesting that BK may be a safe and effective adjuvant in humans.