(EN) A method for treating a virus-containing sample characterized by treating the sample with a treating solution containing (1) an anionic surfactant and (2) any of an amphoteric surfactant, a nonionic surfactant and a protein denaturing agent; a method for assaying a virus by using this treating method; a method for treating a virus-containing sample characterized by treating the sample with a treating solution containing (1) a chaotropic ion and (2) an acidifying agent; a method for assaying a virus by using this treating method; a method for assaying a virus characterized by assaying a virus antigen and a virus antibody in the presence of a surfactant which has alkyl having or more carbon atoms and a secondary, tertiary or quaternary amine and/or a nonionic surfactant on the basis of the bonds to the probes thereof; and a monoclonal antibody for effecting this method and a hybridoma producing the same.