In vitro detection of a specific complex of the bovine leukemia virus (BLV) MA(p15) protein and the 5'-terminal RNA dimer led to the hypothesis that the NH2-terminal domain of retrovirus gag protein precursor is involved in the selective viral RNA packaging mechanism. Here we describe mapping of the BLV RNA for dimer-forming and MA(p15)-binding abilities by a simple cDNA probing method followed by mutation analyses with the reactive U5-5' gag RNA. The RNA dimerization is mediated by the region harboring U5, the primer binding site (PBS), and the 30 bases immediately downstream of PBS. This conclusion is supported by computer-assisted RNA secondary-structure analysis which predicted a multibranched stem-loop folding throughout the dimer region determined. Another region from PBS to the 5'-terminal 60 residues of the gag gene, partially overlapping the dimer region, likely provides essential elements for the MA(p15) binding reaction, although the presence of either the 3' or 5' neighboring sequences increases the complex-forming efficiency significantly, and each of the substructures predicted within the core region has, if any, only very weak affinity to MA(p15). These in vitro characterizations of the BLV RNA may reflect general features of the specific protein-RNA interaction in the packaging events of various retroviruses. 5'-terminal folded structures of retroviral RNA molecules and their biological activities are discussed.