The fungus Agaricus brasiliensis is a Basidiomycete studied because of its immunomodulation and/or antitumor substances. The objective of this study was to verify the Agaricus brasiliensis antineoplasic activity in vivo on different basidiocarp maturation phases on Sarcoma 180 cells implanted in mice. Sarcoma cells were implanted in mice and after seven days mice were divided in three groups. The first group was treated with saline solution, the second group was treated with closed basidiocarp extract solution and the third group was treated with opened basidiocarp extract solution. After 30 days of being daily orally treated with these three solutions all animals suffered euthanasia, and the splenic index, tumor mass and volume were determined. No significant differences of the tumor growth inhibition in function of the different basidiocarp maturation phases for the Agaricus brasiliensis strain were observed. The in vivo basidiocarp antineoplasic average activity was 89.22%.