Viral gene products are generally required in widely differing amounts for successful virus growth and assembly. For coronaviruses, regulation of transcription is a major contributor to these differences, but regulation of translation may also be important. Here, we examine the possibility that the 5' untranslated regions (UTRs), unique for each of the nine species of mRNA in the bovine coronavirus and ranging in length from 70 nucleotides (nt) to 210 nt (inclusive of the common 5'-terminal 65-nt leader), can differentially affect the rate of protein accumulation. When the natural 77-nt 5' UTR on synthetic transcripts of mRNA 7 (mRNA for N and I proteins) was replaced with the 210-nt 5' UTR from mRNA 1 (genomic RNA, mRNA for viral polymerase), approximately twofold-less N, or (N) CAT fusion reporter protein, was made in vitro. Twofold less was also made in vivo in uninfected cells when a T7 RNA polymerase-driven transient-transfection system was used. In coronavirus-infected cells, this difference surprisingly became 12-fold as the result of both a stimulated translation from the 77-nt 5' UTR and a repression of translation from the 210-nt 5' UTR. These results reveal that a differential 5' UTR-directed regulation of translation can occur in coronavirus-infected cells and lead us to postulate that the direction and degree of regulation is carried out by viral or virally induced cellular factors acting in trans on cis-acting elements within the 5' UTR.