(Science: drug) A potassium ionophore antibiotic, produced by streptomyces fulvissimus. Composed of 3 molecules (L valine, D _ hydroxyisovaleric acid, L lactic acid) linked alternately to form a 36 membered ring, that folds to make a cage shaped like a tennis ball seam. This wraps specifically around potassium ions, presenting them with a hydrophilic interior and a lipid bilayer with a hydrophobic exterior. Potassium is thus free to diffuse through the lipid bilayer. Highly ion specific, valinomycin is used in ion selective electrodes.