Using a standard cellular fusion technique and indirect enzyme-linked immunosorbent assay (ELISA), a hybridoma cell line strain secreting anti-HBs monoclonal antibody (mAb) (defined G6 mAb) was obtained. The cells grew and secreted mAb stably. Antibody titers in the culture supernatant and ascites were 2.048×10 and 4.096×10, respectively. By applying the anti-HBs G6 mAb and horseradish peroxidase (HRP)-labeled goat anti-HBs antibody, we developed a sandwich ELISA (defined G6m ELISA) for detecting both wild-type and immune escape mutant HBsAgs (IEM HBsAg). The assay was performed to detect 17 species of genome recombinant expression HBsAg, including two wild-type species and 15 IEM HBsAg species, which varied in the “a” determinant, in a group of patients infected with hepatitis B virus (HBV). The patients previously had a lower ELISA detection signal [(absorbance of patients/absorbance of normal people (P/N): 1.0―4.5)]. The results demonstrated that the sensitivity of this assay to wild-type HBsAg was no less than 0.125g/L; 12 of 15 IEM HBsAg species (P/N≥2.5) were positive for G6 mAb. Of the positive IEM HBsAg species, two had a low absorbance value at 450nm (), one had an intermediate value and nine had a high value, which was 7.55%(mean), 59.4% and 92.1%―109.4% of the wild-type value, respectively. The two species with low value and the three negative species mutated at the bases 120―124 in the first loop of the HBV “a” determinant. Using the G6 ELISA and two commercial ELISA kits (A and B), 177 patients were tested. The G6 ELISA had a significantly higher detection rate than either commercial ELISAs (19.21% 14.89% and 6.21%, respectively; <0.01, <0.05, respectively).