Agaricus blazei Murrill (ABM) popularly known as 'Cogumelo do Sol' in Brazil, or 'Himematsutake' in Japan, is a mushroom native to Brazil and widely cultivated in Japan for its medicinal uses and is now considered one of the most important edible and culinary-medicinal biotechnological species. This study is the first tumor growth model to evaluate the amelioratory effect of ABM extract using HT-29 human colon cancer cells in severe combined immunodeficiency (SCID) mice. Forty SCID mice were inoculated with HT-29 cells to induce tumor formation and were then divided into four groups. All the four groups (control, low, medium and high concentration treatment) of mice were separately orally administered 0 mg, 1.125 mg, 4.5 mg or 45 mg ABM extract daily. After six weeks of treatment, 8 out of the 40 mice had not survived including one mouse which scored +++ (tumor up to 15 mm diameter) and four mice which scored ++++ (tumor over 15 mm diameter) in the control group and three mice which scored ++++ on the low-dose ABM treatment. After high- or medium-dose treatment, all ten mice in each group survived. The oral administration of ABM does not prevent tumor growth, as shown by increased tumor mass, but compared with the control group, the tumor mass seems to grow more slowly depending on the ABM dose.