The genome of the defective avian tumor virus MH2 was identified as a RNA of 5.7 kilobases by its presence in different MH2-helper virus complexes and its absence from pure helper virus, by its unique fingerprint pattern of RNase T1-resistant (T1) oligonucleotides that differed from those of two helper virus RNAs, and by its structural analogy to the RNA of MC29, another avian acute leukemia virus. Two sets of sequences were distinguished in MH2 RNA: 66% hybridized with DNA complementary to helper-independent avian tumor viruses, termed group-specific, and 34% were specific. The percentage of specific sequences is considered a minimal estimate because the MH2 RNA used was about 30% contaminated by helper virus RNA. No sequences related to the transforming src gene of avian sarcoma viruses were found in MH2. MH2 shared three large T1 oligonucleotides with MC29, two of which could also be isolated from a RNase A- and T1-resistant hybrid formed between MH2 RNA and MC29 specific cDNA. These oligonucleotides belong to a group of six that define the specific segment of MC29 RNA described previously. The group-specific sequences of MH2 and MC29 RNA shared only the two smallest out of about 20 T1 oligonucleotides associated with MH2 RNA. It is concluded that the specific sequences of MH2 and MC29 are related, and it is proposed that they are necessary for, or identical with, the onc genes of these viruses. These sequences would define a related class of transforming genes in avian tumor viruses that differs from the src genes of avian sarcoma viruses.