Abstract: The present invention provides a method for identifying a substance capable of inhibiting at least one activity of an RNA-dependent RNA polymerase (RdRp) comprised in an influenza viral ribonucleoprotein (RNP) complex, which method comprises or consists of: (a) providing an aqueous suspension of purified influenza virus comprising said RNP complex; (b) adding to said suspension of step (a) (i) a nonionic or zwitterionic surfactant; and (ii) a reducing agent; (c) incubating the mixture obtained in step (b) in order to obtain an influenza virus lysate; (d) contacting the influenza virus lysate obtained in step (c) with a test substance under conditions that permit said test substance to interact with said RdRp comprised in said RNP complex, said RNP complex being present in said influenza virus lysate; and (e) determining whether said test substance inhibits said at least one activity of said RdRp.