5-(Iodoacetamido)fluorescein is a thiol reactive fluorescein derivative for synthesis of fluorescently labeled organelles, proteins and peptides at or near their physiological pH value (pH 7) including enzymes (e.g. (Na, K)-ATPase) or organelles like nuclei and nuclear matrices.Fluorescence:λex 492 nm; λem 515 nm (Mercaptoethanol adduct);λex 493 nm; λem 522 nm (pH 9.2)Thiol-Reactive fluorescent probes such Iodoacetamides or maleimides are used for preparation of green-fluorescent thiol conjμgates of biomolecules. 5-(Iodoacetamido)fluorescein is a thiol reactive fluorescein derivative for synthesis of fluorescently labeled organelles, proteins and peptides at or near their physiological pH value (pH 7) including enzymes (e.g. (Na, K)-ATPase) or organelles like nuclei and nuclear matrices.Fluorescence:λex 492 nm; λem 515 nm (Mercaptoethanol adduct);λex 493 nm; λem 522 nm (pH 9.2) Thiol-Reactive fluorescent probes such Iodoacetamides or maleimides are used for preparation of green-fluorescent thiol conjugates of biomolecules.