Extracts of Agaricus blazei Murill have been found to have anti-tumor activity. In particular, a polar, high molecular weight fraction of an Agaricus blazei Murill extract has been found which has an inhibitory effect on legumain activity. Extracts of Agaricus blazei Murill can be used in a pharmaceutical composition to inhibit cysteine protease legumain in a patient suffering from a tumor thereby treating the tumor and preventing new tumors.