Porcine reproductive and respiratory syndrome virus (PRRSV) MN-1b strain open reading frame 4 (ORF4) has been cloned, sequenced, and expressed in Escherichia coli. The homologies of nucleotide and amino acid sequences between MN-1b (US isolate) and LV (European isolate) are 69% and 64%, respectively. The data also showed that ORF4 of MN-1b is 36 bases shorter than that of LV. Western blot analysis of expressed recombinant ORF4 protein reacted with 65% (26/40) of PRRSV-infected pig sera tested. These results demonstrated that ORF4 of PRRSV may not be a well-conserved region.