OsK1 is a 38-residue peptide with 3 disulphide bridges and is found in the venom of the scorpion Orthochirus scrobiculosus. It is potently active on voltage-gated K+ channels Kv1.1, Kvl.2 and Kvl.3, and moderately active on the type 1 intermediate-conductance Cat2+­-activated channel Kca3.1. Derivatives of OsK1, particularly involving truncation or point mutations, have been developed to enhance the activity against and selectivity for the Kvl.3 channel. This renders the derivatives likely candidates for the treatment of autoimmune diseases, including multiple sclerosis. Such use may be alone or in combination therapy with maurotoxin, another scorpion toxin.