Genome sequences of two recent field isolates of avian leukosis viruses in the DNA of normal and neoplastic chicken cells were studied by DNA-RNA hybridization under conditions of DNA excess. Comparisons were made between 60-70S RNA from these viruses and that of a chicken endogenous type C virus (RAV-0), and of a series of "laboratory" leukosis and sarcoma viruses, by competitive hybridization analysis. A minimum of 18% of the genome sequences of both ALV isolates detected in DNA from lymphomas they induced were not detected in normal chicken DNA. The vast majority of the fraction of RNA sequences from ALV which do form hybrids with normal chick DNA appear to be reacting with the endogenous provirus of RAV-0. The genomic representation of a variety of avian leukosis and sarcoma viruses in normal chicken cells could not be distinguished by these methods (except that 13% of the RAV-0 genome was not shared with any of the other viruses). In contrast, the portion of the ALV genome exogenous to the normal chicken geome showed significant divergence from that of two sarcoma viruses (Pr RSV-C and B-77). The increased hybridization of ALV RNA with lymphoma DNA was used to detect the appearance of ALV specific sequences in the bursa of Fabricius following infection.increased hybridization was correlated with both the time after infection and the extent of replacement of the bursa by lymphoma. About one half of the increase in hybridization preceded histologic evidence of transformation.