This invention provides a monoclonal antibody which recognizes an epitope corresponding to amino acids 10 to 220, amino acids 221 to 297, or amino acids 469 to 662, counting from the N-terminus of a human Mx protein MxA, and specifically reacts with the human Mx protein by western blotting, immunoprecipitation or immunocyte staining, and a hybridoma which produces the antibody. The human Mx protein MxA monoclonal antibody of this invention can be used, for example, in the diagnosis of viral infection.