Here, the effect of oral administration of the extract of sangwhang mushroom (Phellinus linteus, PL) grown on germinated brown rice on lung metastasis of tumor cells was examined in mice, and a possible immunological action was suggested. Oral administration of the extract of sangwhang mushroom grown on germinated brown rice (PB, 90 mg/kg) prior to tumor inoculation significantly inhibited experimental lung metastasis produced by colon 26-M3.1 carcinoma cells. Also oral administration of PB after tumor inoculation significantly inhibited experimental lung metastasis of tumor cells. In addition, oral administration of PB significantly suppressed tumor growth as well as tumor-induced angiogenesis. However, neither PL nor germinated brown rice (BR) was active. Oral administration of PB, but not PL and BR, resulted in a significant enhancement of natural killer (NK)-mediated killing activity against tumor cells. These results suggest that oral administration of PB can induce antitumor activity to inhibit tumor metastasis, and its antitumor effect is associated with NK cell activation.