Both the aqueous extracts of fruit bodies of about 1,000 species of higher fungi and shake-flask culture filtrates of the isolated mycelial strains have been screened for the antitumor activities using Ehrlich ascites tumor. About 14% of the aqueous extracts of the sporophores and 8% of the culture filtrates of the mycelia proved to show the tumor-inhibiting properties. During the screening course, it was found that high molecular substances with antitumor activities were widely distributed among the aqueous extracts or culture filtrates¹⁾. In 1957, Lucas et al.²⁾ reported a tumor-inhibiting substance identified as either a peptide or protein from the mushroom Boletus edulis. In 1960, an antitumor antibiotic, calvacin, was isolated from Calvatia gigantea and identified as a nondiffusible, basic mucoprotein³⁾. The authors have isolated a basic protein, named flammulin, from a basidiomycete, Flammulina velutipes (Fr.) Sing. belonging to the genus Flammulina of Tricholomataceae. In this paper the isolation and some chemical and biological properties of flammulin are described.