To obtain a new monoclonal antibody specifically reacting with Streptococcus mutans in an antibody-excess region to form a plateau on a volume- action curve of an antigen-antibody reaction and useful for the prevention, treatment, diagnosis, research, etc., of the cariogenesis in the oral cavity.

This new monoclonal antibody is specifically reactive with Streptococcus mutans (serum type: c/e/f) in an antibody excess region to form a plateau on a volume-action curve of an antigen-antibody reaction and non- reactive with Streptococcus mitis, Streptococcus salivarius and Streptococcus sanguis. It is effective for the prevention and treatment of cariogenesis in the oral cavity and useful also as a basis for the diagnosis and research of the caries in the oral cavity. The monoclonal antibody can be produced by immunizing a BALB/c mouse with a protein antigen separated from Streptococcus mutans, collecting the spleen cells from the mouse, fusing with a myeloma cell, monoclonizing by clone selection and culturing the obtained hybridoma.