Employing enzymatic reactions in vitro, we have identified the presence of oligoribonucleotides at the 5' end of strong-stop plus [(+)] DNA. Similar results were obtained whether the strong-stop (+) DNA was synthesized by preparations of detergent-disrupted avian sarcoma virus or reconstructed reactions containing purified reverse transcriptase and a template that mimics the purported natural template for strong-stop (+) DNA synthesis. The latter reactions provide a system to delineate more precisely the discrete requirements necessary for the initiation and synthesis of this species of (+) DNA.