Oregonin, a diarylheptanoid derivative from Alnus hirsuta Turcz, Betulaceae, was evaluated for its antitumor activity. Oregonin, known to have an antitumor function, and is a novel immunomodulator, which may augment macrophage activity. MTT assays and NO production tests were performed in order to investigate the cytotoxicity of oregonin in tumor cells and to examine its influence on macrophage in detail. In this study, the tumoricidal activity was also evaluated by a MTT assay. The cytotoxicity measurements in the oregon in-treated group both in vitro and in vivo showed a significant difference from that of the control group. In vivo, oregonin significantly increased NO production in a dose-dependent manner, and in vitro, the thioglycolate-induced inflammatory macrophages increased NO production in a dose-dependent manner after incubation. These results suggest that oregonin reacts with both the inflammatory and non-inflammatory macrophages in a similar way.