Fluorescein-12-ddUTP can be used for 3’-end labeling of oligodeoxynucleotides using terminal transferase. It also serves as a substrate for E.coli DNA Polymerase I (holoenzyme and Klenow fragment), T4 and T7 DNA polymerases, Taq DNA polymerase and reverse transcriptase. Oligonucleotide probes labelled with fluorescein-12-ddUTP can be detected by direct fluorescence detection (green fluorescence) or by methods using horseradish peroxidase- or alkaline phosphatase-linked anti-fluorescein antibodies.