A 5-carboxyfluorescein (FAM) phosphoramidite reagent has been synthesized for use on automated DNA synthesizers, to prepare fluorescent dye labelled oligonucleotides. Oligonucleotides can be labelled at any site in a sequence and multiple labelled. The FAM dye can also be coupled to a polystyrene synthesis support to prepare 3' fluorescent labelled oligonucleotides.