Recently, Rhei Rhizoma extracts (RRE) have begun to receive more attention as potential biological response modifiers. In the present study, we studied the antiproliferative effect of RRE on tumor cells and the effect of RRE on macrophage function. A variety of tumor cells and macrophages were treated with RRE at various concentrations. The effect of RRE on cell proliferation was measured by MTT assay and the effect of RRE on the production of nitric oxide (NO) was determined in the macrophage-like cell lines Raw264.7, C6 and peritoneal macrophages (pMQ). RRE inhibited the growth of tumor cells (e.g., B16, HOS). However, the effects of RRE on the production of NO varied with macrophage types. RRE had no effect on C6 cell growth and slightly increased the growth of Raw264.7 cells. In addition, treatment of normal pMQ with RRE enhanced NO production in a concentration-dependent manner, whereas RRE suppressed NO production at $50;{mu}g/mL$ in both Raw264.7 and C6 cells. However, RRE suppressed NO production in LPS/IFN-$gamma$-stimulated C6 cells. Overall, these results suggest that RRE elicits an antiproliferative property and differentially modulates NO production in various macrophages, and have a potential for therapeutic application.