Total protein was extracted from the single leave and fruit of untransformed control and transgenic plants as previously described (Mason et al., 1992). Each group contained 20 plants. The extracts were clarified and assayed in triplicates for the levels of HBsAg expression, and the mean values were taken as an estimate of the amount of HBsAg. An HBsAg enzyme-linked immunosorbent assay (ELISA) kit was purchased from Wantai Biotechnology Co., Ltd. (Beijing, China), and HBsAg standards were provided by Huamei Biotechnology Company. The positive control (human serum derived HBsAg) as a standard and negative