Paeonia japonica var. pilosa $N_{AKAI}$, (PJ; Baek-Jak-Yak) is a medicinal plant which has been widely used as a component or blood-building decoctions. This study was performed to investigate the immunomodulative effects of PJ in mice, using in vitro and in vivo experiments. The immunomodulative effects were studied in vitro by determining the proliferation or mice splenocytes and the production of three kinds of cytokines (IL-1$eta$, IL-6, TNF-$alpha$) by mire peritoneal macrophages which were cultured with sequential fractions of PJ methanol extract (methanol, hexane, chloroform, ethylacetate, butanol and water). In an in vivo experiment using mice, different concentrations of PJ water extract were orally administrated every other day for two weeks. The production of cytokines (IL-1$eta$, IL-6, TNF-$alpha$) secreted by activated macrophages, and the proliferation of mice splenocytes, were used as indices for immunocompetence. In vitro supplementation using a hexane fraction of PJ in the range of 1 to 100 $mu$ g/ml enhanced splenocyte proliferation by 1.8 to 12%, and by 10-15% using an aqueous fraction, compared to the control. IL-l$eta$ production was significantly increased with the supplementation of butanol, hexane and water extracts of PJ Higher levels of IL-6 production were detected with supplementation of chloroform or water extracts. However, there were no significant differences in the production of TNF-$alpha$ among the treated groups and the control. From the in vivo study, the highest proliferation of splenocytes was seen in the mice orally administrated with the PJ water extract at the concentration of 500 mg/kg body weight. In the case of cytosine production, IL-1-$eta$, IL-6, and TNF-$alpha$ released by activated peritoneal macrophages were augmented by the oral administration of a PJ water extract. These results indicate that Pl may enhance the immune function by regulating splenocyte proliferation and cytokine production capacity in mice.