In experimental lung metastasis of colon26-M3.1 carcinoma cells, we found that the prophylactic or therapeutic admini-stration of Inonotus obliquus extracts significantly inhibited lung metastasis. In an in vitro cytotoxicity analysis, the extracts did not affect colon26-M3.1 cell growth at concentrations up to 1000 ${mu}g/m{ell}$. Peritoneal macrophages that were stimulated with the extracts produced $TNF-{alpha}$. These data suggest that Inonotus obliquus extract has antitumor activity to inhibit tumor metastasis, and its antitumor effects are partially associated with macrophages activation.