Biotin-Fluorescein is a heterobifunctional reagent containing a biotin moiety and a fluorescein group separated by a long-chain hydrophilic polyethylene glycol (PEG) spacer arm.

Biotin-Fluorescein is used to label biotin binding protein molecules including avidin and streptavidin, which each have four biotin-binding sites. The resulting fluorescent biotin-binding protein can then amplify a signal generated in assays that use avidin-biotin detection systems.