A map of the large T1 oligonucleotides of the RNA of Prague Rous sarcoma virus, strain B (Pr RSVb) has recently been established (Coffin and Billeter, submitted for publication). Since the RNA of Rous associated virus, type 1 (RAV-1) lacks many of the large 1 oligonucleotides of Pr RSV-B and contains others not present in the latter, the RNA of recombinants between RAV-1 and Pr RSV-B could be analyzed with regard to the origin of its sequences. Recombinants were selected for transforming capacity (characteristic for Pr RSV-B) and ability to grow on C/B chicken fibroblasts (characteristic for RAV-1). Four out of five recombinants examined had undergone at least two crossovers. The set of Pr RSV-B-specific oligonucleotides present in all recombinants defined an RNA region near the poly(A) segment; this must contain genetic information required for transformation required for transformation (the onc function). All recombinants lost a set of contiguous Pr RSV-B-specific oligonucleotides and concomitantly acquired a set of RAV-1-specific oligonucleotides. These define a region in the middle section of the oligonucleotide map, all or some of which must be required for determining growth capacity on C/B cells (the env function).