Amine-reactive fluorescent probe for introducing a fluorescein label at a nucleophilic residue of a target molecule. This fluorescein labeling reagent has an added (X) spacer to improve accessibility of the reactive group by nucleophiles in preparation of bioconjugates. The reactive N-hydroxysuccinimide (NHS) group can be used for easy attachment to proteins or amine containing nucleotides using simple procedures. Spectral properties: lambdaex 490 nm; lambdaem 514 nm in DMF (pH 8.2) and lambdaex 491 nm; lambdaem 515 nm in 0.1 M Tris pH 9.0.