A synthetic HDAg 27 peptide which was selected and designed by the authors and synthesised by Shanghai Institute of Biochemistry, Chinese Academy of Science was used with ELISA method to detect serum anti HD in HBV infected subjects in Chongqing. Anti HD was positive in one of 300 blood donors and was negative in all of 113 cases of hepatitis A and 58 cases of hepatitis non-B. Anti HD was positive in 106 out of 882 cases with positive HBV marker (12.02%), among which anti HD was positive in 3.17% (13/410) of HBsAg carrier, 14.4% (11/76) of acute hepatitis, 7.6% (1/13) of chronic persistent hepatitis, 17.68% (22/121) of chronic active hepatitis, 19.77% (17/86) of severe hepatitis, 29.49% (23/78) of liver cancer and 19.39% (19/98) of primary hepatic cancer. These results coincided with those of previous reports. The coincidence rate was 94.9% (74/78) when compared with Abbott EIA kit. When the natural HDAg was used to compete anti HD in four anti HD positive and two anti HD negative serum specimens, anti HD was negative in all specimens. It is shown that the HDAg 27 peptide has natural HDAg activity capable of being recognized by natural anti HD and is a new diagnostic agent being more simple, save, stable and reliable.