In the course of screening for immunomodulators modulating T cell functions, we found a novel, low MW immunomodulator named cytoblastin in a culture filtrate of the strain, MI43-37F11 which was isolated from the soil sample collected in Yokohama City, Kanagawa Prefecture, Japan. The strain was identified as Streptoverticillium eurocidicum on the basis of its cultural properties. Cytoblastin showed low cytotoxicity and no antimicrobial activity and promoted the proliferation of T cells. In this communication, the production, isolation, physicochemical properties, structure and biological properties are reported.