Lymphostin (LK6-A), a novel immunosuppressant, was isolated from the culture broth of Streptomyces sp. KY11783. Lymphostin showed potent inhibitory activity against the lymphocyte kinase, Lck (p56(lck)). The structure of lymphostin was determined by spectroscopic methods to be a novel tricyclic aromatic alkaloid. A key feature of the structure is the pyrrolo[4,3,2-de]quinoline moiety which is rare among natural products.