The invention relates to a substrate and method for the assay of ribonuclease H activity. The substrate comprises a DNA/RNA heteroduplex molecule having fluorophore and quencher molecules in close proximity, which emit a background level of fluorescence in the absence of ribonuclease H activity due to quenching of the fluorophore as a result of fluorescence resonance energy transfer. Ribonuclease H cleavage of the heteroduplex results in an increase in fluorescence emission due to the destabilization and separation of the DNA and RNA molecules and their associated fluorophore and quencher molecules. The assay method of the present invention is suitable for use in screening (e.g. high throughput screening) for modulators, e.g. inhibitors, of the ribonuclease H activity associated with, for example, reverse transcriptase and ribonuclease H enzymes.