The mRNA guanyltransferase-mRNA methyltransferases of vaccinia virions can be used to introduce a 5'-terminal m7g(5')pp(5')Apm... capping group onto the RNA of satellite tobacco necrosis virus (STNV RNA) to yield intact capped STNV RNA. Studies with an in vitro system from wheat germ and limiting quantities of capped and uncapped STNV RNA show that the rates and extents of formation of initiation complexes of protein synthesis by intact capped and uncapped STNV RNA are identical, suggesting that 5'-terminal cap groups cannot function in the translation of STNV RNA. Also, the cap analogue pm7G equally inhibits the initiation and the translation of limiting quantities of both capped and uncapped STNV RNA. These contrasting observations suggest that the wheat germ system contains a pm7G sensitive protein and that STNV RNA has a tertiary structure that restricts the function of an added 5'-terminal capping group. This theory is supported by observations that fragmented capped STNV RNA is better at forming initiation complexes than is equally fragmented uncapped STNV RNA.