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Purpose of work：The non-structural protein 4 (Nsp4)of porcine reproductive and respiratory syndrome virus(PRRSV) functions as a 3C-like proteinase (3CLpro)and plays a pivotal role in gene expression andreplication. We have examined the biochemical prop-erties of PRRSV 3CLpro and identified those aminoacid residues involved in its catalytic activity as aprelude to developing anti-PRRSV strategies. The 3C-like proteinase (3CLpro) of porcine repro-ductive and respiratory syndrome virus (PRRSV) wasexpressed in Escherichia coli and characterized. The optimal temperature and pH for its proteolytic activitywere 8℃ and 7.5, respectively. Na+(1000 mM) andK+(500 mM) were not inhibitory to its activity butCu2+, Zn2+, PMSF and EDTA were significantly inhibitory. His39, Asp64 and Ser118 residues wereidentified to form the catalytic triad of PRRSV 3CLproby a series of site-directed mutagenesis analysis.