This study was undertaken to investigate the in vivo anti-tumor activity of MGN-3/Biobran (also known as Lentin Plus 1000), a modified arabinoxylan rice bran. Swiss albino mice were inoculated intramuscularly in the right thigh with Ehrlich ascites carcinoma (EAC) cells. On Day 8, mice bearing a solid Ehrlich carcinoma (SEC) tumor were treated with MGN-3 via intraperitoneal injection. Tumor growth, cytokine production, and apoptotic effect of MGN-3 were examined. MGN-3 caused a highly significant delay in both tumor volume (63.27%) and tumor weight (45.2%) as compared to controls (P < 0.01). The mechanisms by which MGN-3 exerts its antitumor effect seem to involve its ability to induce apoptosis and immune modulation. MGN-3 induced a 1.8-fold increase in the percentage of apoptotic SEC cells as determined by flow cytometry and the histopathological examination. In addition, MGN-3 influenced plasma cytokine production by increasing the levels of tumor necrosis factor-α and interferon-γ , while downregulating levels of the immune suppressing cytokine interleukin-10. Data also showed that non-tumor-bearing mice intramuscularly injected with MGN-3 resulted in a twofold increase in natural killer activity. No adverse side effects due to MGN-3 treatment were observed; all animals displayed normal feeding/drinking and life activity patterns. These data may have clinical implications for the treatment of solid cancers.