We identified the critical amino-acid residues in antigen M derterminant(MAD) epitope of human cytomegalovirus protein M. On the basis of the peptide sequence of MAD, some conservative residues were mutated into the glycine residue. Then the gene fragment of mutants linked to amino terminal of Fc were cloned into the plasmid pET32-Fc and expressed by fusion with Fc. After purified by protein A affinity chromatography, the activity of mutants binding the goat polyclonal antibodies against human cytomegalovirus(HCMV) were detected by ELISA and Western blotting. Our results showed that when glutamine residue was mutated into glycine residue, the activity of MADQ→G binding the goat polyclonal antibodies against HCMV was reduced apparently. Other mutants did not have the same characteristics. The activity of MAD was closely related to the conformation of glutamine residue.