(EN) A method for treating a hepatitis C or hepatitis B virus-containing sample, characterized by treatment of the virus-containing sample with a treatment solution containing (1) an anionic surfactant and (2) an amphoteric surfactant, nonionic surfactant or protein denaturant, and a virus assay method using said treating method. Also provided is a method for treating a hepatitis C virus-containing sample, characterized by treatment of the virus-containing sample with a treatment solution containing (1) a chaotropic ion and (2) an acidifying agent, and a virus assay method using said treating method. Further provided is a hepatitis C virus assay method comprising treating a hepatitis C virus antigen-containing sample with a surfactant having an alkyl group of 10 or more carbon atoms and a secondary, tertiary or quaternary amine, or a nonionic surfactant, or of both of them, and measuring binding of the antigen with a hepatitis C virus antibody. Finally, there is also provided a monoclonal antibody and a hybridoma producing the same for carrying out the above methods.