To test the antiviral activity of siRNA, BHK-21 cells cultured in 96-well plates were transfected with siRNA-expressing plasmids individually. As controls, the cells were either not treated or treated with plasmid pNTH21 or pLacZ. After 24 h, the cells were infected with 100 TCID 50 of CHA/99, HNK/2002, YNBS/ 58 or Jiangsu/2005. The cells were observed continuously under the microscope. Microscopy examination revealed that the cells treated with p3D-NT56 or p2B-NT25 had delayed cytopathic effect (CPE) development when challenged with either serotype O or Asia I of FMDV. However, the control cells, either blank or transfected with the control plasmids, showed an extensive CPE within 24 h post infection (p.i.). Furthermore, we investigated the effect of plasmid-mediated RNAi on FMDV replication by measuring the TCID 50 of supernatants of lysed cells, which were collected at 12, 24 and 48 h after viral challenge. Consistent with the observation, p3D-NT56 and p2B-NT25 showed a wider cross-inhibitory effect than the other plasmids, and these 2 plasmids inhibited 4 FMDV isolates (Fig. 1) . The supernatant TCID 50 for CHA/99, HNK/2002, YNBS/58 and Jiangsu/2005 of the cells treated with p3D-NT56 or p2B-NT25 was reduced by 30-to 300-fold at 48 h post infection (Fig. 1 ) compared to the control groups.