The positive-strand RNA genomes of caliciviruses are not capped, but are instead covalently linked at their 5' ends to a viral protein called VPg. The lack of a cap structure typical of eukaryotic mRNA and absence of an internal ribosomal entry site suggest that VPg may function in translation initiation on calicivirus RNA. This hypothesis was tested by analyzing binding of Norwalk virus VPg to translation initiation factors. The eIF3d subunit of eIF3 was identified as a binding partner of VPg by yeast two-hybrid analysis. VPg bound to purified mammalian eIF3 and to eIF3 in mammalian cell lysates. To test the effects of the VPg- eIF3 interaction on translation, VPg was added to cell-free translation reactions programmed with either capped reporter RNA, an RNA containing an EMCV internal ribosomal entry site (IRES) or an RNA with a cricket paralysis virus IRES. VPg inhibited translation of all reporter RNAs in a dose-dependent manner. Together, the data suggest that VPg may play a role in initiating translation on calicivirus RNA through unique protein-protein interactions with the translation machinery.