Peptide toxins have enormous potential as therapeutic leads.Here I describe the progress of analogues based on one such toxin,ShK,to clinical trials for autoimmune diseases.Effector memory T cells,which are involved in autoimmune diseases such as multiple sclerosis,type 1 diabetes mellitus and rheumatoid arthritis,express Kv1.3 potassium channels,which play a major role in their activation.Blockers of lymphocyte Kvl.3 channels preferentially inhibit the activation of these cells and therefore show considerable potential as therapeutics for autoimmune diseases.