An indirect enzyme-linked immunosorbent assay (iELISA) method was established to evaluate specific antibody against M protein. A truncated M protein (85-174aa), which was used as the coating antigen, was expressed and purified using a prokaryotic expression system. The truncated M gene cDNA was amplified from pMD-M using the truncated-M-F and truncated-M-R primers (Table 1) , ligated into pGEX-6P-1 (GE Healthcare), and subsequently named pGEX-M. For protein expression, pGEX-M was transformed into E.coli BL21 (DE3) and recombinant protein expression was induced with 0.5 mM IPTG. The samples were harvested by centrifugation and the pellets were resuspended with phosphate buffered saline (PBS, PH 8.0). After being analyzed by sodium dodecyl sulfatepolyacrylamide gel electrophoresis (SDS-PAGE) and Western blot with anti-PRRSV-M monoclonal antibody (our laboratory collection), the fusion-expressed truncated M protein was purified by GST tag according to the manufacturer's instructions (GE Healthcare). Finally, the concentration of the purified protein was determined using a Bradford kit (Bio-Rad) according to the manufacturer's instructions. 