A composition of an immunosuppressant protein (HISP) which is achieved by the steps of obtaining supernatant from hNT neuronal cells; exposing the supernatant to preparative polyacrylamide gel; placing the active isoelectric fraction on a Blue Sepharose column to bind albumin; and collecting the free fraction containing the concentrated, isolated HISP. The HISP is anionic, has a molecular weight of 40-100 kDa, an isoelectric point of about 4.8 and is obtained from the supernatant of hNT cells. HISP can suppress proliferation of responder peripheral blood mononuclear cells in allogeneic mixed lymphocyte cultures; HISP can suppress T-cell proliferation and IL-2 production in response to phorbol 12-myristate 13-acetate (PMA), ionomycin and concanavalin-A. HISP does not act through the T-cell receptor-CD3 complex or via altered accessory signal cells.