The interactions between Rous Sarcoma virus RNA and the viral proteins have been analysed in the virus and in vitro. We show that it is protein Pl2 and not Pl9 that binds tightly to RSV RNA both in the virus and in vitro. Specific RNA sequences are recognized by protein Pl2 in the virus and such sequences are close to or involve the splice sites, and the dimer linkage site likely to be required for viral packaging. In vitro Pl2 does not impair proviral DNA synthesis whereas it can completely inhibit RNA translation. Such data suggest that Pl2 may control negatively viral RNA splicing and translation and positively the packaging process.