Abstract: A thirty-nine amino acid peptide, Margatoxin (MgTX), is purified to homogeneity from venom of the scorpion Centruroides margaritatus. The gene encoding MgTX is constructed and this gene is expressed in E. coli, to produce recombinant MgTX. MgTX is a potent and selective inhibitor of a voltage-dependent K.sup.+ channel present in human lymphocytes. MgTX exhibits immunosuppressant activity with human T-lymphocytes, and is useful as an immunosuppressant, in modeling nonpeptidyl K.sup.+ channel blockers, and in establishing biochemical assays based on ligand binding or other protocols with which to screen for other novel modulators of voltage dependent K.sup.+ channels in lymphocytes and other tissues including the brain. As an immunosuppressant, MgTX is useful in the treatment of autoimmune diseases, the prevention of rejection of foreign organ transplants and/or related afflictions, diseases and illnesses.