5(6)-Carboxyfluorescein N-hydroxysuccinimide ester is a fluorescent probe useful for the preparation of tubulin derivatives. It can also be used to determine this product’s own presence in plasma and serum by fluorescence polarization immunoassay (FPIA). 5(6)-Carboxyfluorescein N-hydroxysuccinimide ester reacts with primary and secondary amine groups on proteins and other biomolecules.