In vitro degradation of antisense oligonucleotides protected or not on their 3′ side against enzymatic attack by a naturally forming hairpin has been studied by fluorescence resonance energy transfer (FRET). The two oligonucleotides d(5′TTCTCGCGAAGC3′) forming the hairpin and d(5′TTCTCCGGAAGC3′) as a control were labeled on their 5′ side by tetramethylrhodamine and on their 3′ side by fluorescein. Fluorescein has been shown not to hinder the hairpin formation and to give an additional protection against nucleases. The FRET technique proved adequate for an in situ study of these protected antisense oligonucleotides in living cells.