Mouse hepatitis virus (MHV) gene 5 contains two open reading frames. We have expressed the second open reading frame of this gene (gene 5 ORF 2) in an Escherichia coli expression system. This system utilized a plasmid which contained the promoter and the first 36 codons of the recA gene fused in frame with the MHV gene 5 ORF 2, which is fused in turn to the ฮฒ-galactosidase gene. The protein product of this gene fusion was used to raise antibody to gene 5 ORF 2. The specificity of the antibody was verified by immunoprecipitation of the in vitro transcribed and translated protein product of gene 5 ORF 2. The second reading frame of MHV gene 5 was shown to be expressed during the course of infection by immunocytochemistry and radioimmunoprecipitation using the antibody raised against the E. coli fusion protein and by two-dimensional gel electrophoresis. ยฉ 1988.