Lymphostin (LK6-A), a novel immunosuppressant, was isolated from the culture broth of Streptomyces sp. KYI 1783. 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.