Abstract: This invention provides anti-HBs monoclonal antibodies HBs 8Cl, HBs 18E9 and HBs 22B7, an anti-human IgM monoclonal antibody HIgM 10C9 and an anti-mumps virus monoclonal antibody MPV 10G3 capable of inhibiting nonspecific reaction in the reverse passive agglutination, hybridomas producing them, a method for sensitizing fixed hemocytes with anti-HBs monoclonal antibodies, a fixed hemocyte sensitized with them by the said method, a method for inhibiting the said nonspecific reaction by utilizing the said monoclonal antibodies and an assay for HBs antigen by appropriately combining the above.