A method is provided for making synthetic capped RNAs. These compounds serve as substrates for the virally encoded endonuclease associated with influenza virus. We are able to assay for this unique and specific viral activity of cleavage of a capped RNA in vitro. Therefore, screening of inhibitors of this activity is possible. In addition, short non-extendible (due to their length or because of the modification of the 3'-end of the oligo, i.e. 3'-dA) RNAs are potent inhibitors of the cleavage of capped RNAs by influenza endonuclease. Finally, these compounds may be used to investigate viral and cellular mechanisms of transcription/translation or mRNA maturation.