Interferon action against viruses is mediated in part through a ribonucleic acid (RNA) decay pathway known as the 2-5A system. Unusual 5'-triphosphorylated, 2',5'-linked oligoadenylates (2-5A) are produced in mammalian cells by interferon-inducible 2-5A synthetases (OAS) in response to viral double-stranded RNA. 2-5A activates a uniquely regulated endoribonuclease, RNase L, resulting in the cleavage of single-stranded viral and cellular RNAs, thus suppressing viral replication. In addition, RNase L was recently identified as a strong candidate for the hereditary prostate cancer 1 susceptibility allele. RNase L is ubiquitously expressed at basal levels in a wide range of mammalian cell types. Conventional RNase L assays, which can be inconvenient and cumbersome, typically involve cleavage of radioactively labeled RNA species or of endogenous ribosomal RNA. Here we describe a convenient, rapid, nonradioactive, and relatively inexpensive fluorescence resonance energy transfer (FRET) that may be used to accurately measure levels of either 2-5A or RNase L activity with a high degree of specificity and sensitivity. The RNA probe used in the FRET assay was designed based on a region of respiratory syncytial genomic RNA. We demonstrate the utility of our FRET assay with several novel biostable analogs of 2-5A.