Abstract: Novel capped oligonucleotides useful in treatment of influenza infection. A synthetically derived 67-nucleotide RNA substrate, containing a ?.sup.32 P! labeled cap-1 structure was used to analyze parameters of influenza virus endonuclease activity. This substrate was specifically cleaved by the influenza virus polymerase to yield a single capped 11-nucleotide fragment capable of directly priming transcription. An analysis of systematic truncations of this RNA substrate in cleavage, elongation, and binding reactions demonstrated that the minimum chain length required for cleavage was one nucleotide past the cleavage site. In contrast, the minimum chain length required for priming activity was found to be 9 nucleotides, while a chain length of at least 4 nucleotides was required for efficient binding.