This ShK analog containing a fluorescein-6-carboxyl group attached through an {2-[2-aminoethoxy]ethoxy}acetic acid (AEEAc) linker to the ƒ-amino group of Arg¹, selectively blocked the voltage gated T-lymphocyte K⁺ Kv1.3 channels implicated in the pathogenesis of experimental autoimmune encephalomyelitis. It potently blocked Kv1.3 channels at picomolar concentrations and exhibited >80 fold specificity for Kv1.3 over Kv1.1 and other Kv channels. In flow cytometry experiments, chronically activated rat and human T-lympocytes with >600 Kv1.3 channels/cell were readily distinguished from resting and acutely activated T-lymphocytes with lower Kv1.3 channel numbers. 6-FAM-AEEAc-Stichodactyla helianthus neurotoxin (ShK) may therefore be a useful tool to detect the presence of T-lymphocytes with high expression of Kv1.3 channels in normal and diseased tissues.