Bovine leukemia virus (BLV) single-stranded cDNA was used to study the distribution of DNA sequences in tissues (normal or malignant) from bovine, ovine and human origin. After recycling against normal bovine DNA, BLV (3H) cDNA hybridized with bovine enzootic tumor DNA but did not hybridize with normal bovine DNA. These results indicate that BLV is an exogenous RNA oncogenic virus and confirm that enzootic bovine leukosis (EBL) is an infectious disease. Proviral BLV sequences were also detected in buffy coat cells of animals in persistent lymphocytosis (PL) and carrying antibodies to BLV but no tumors. In animals at the tumor stage of EBL, the proviral sequences were found in buffy coat cells, in solid tumors (lymphosarcomas) and in organs infiltrated with tumoral lymphoid cells but not in apparently normal organs. No hybridization above background was observed between BLV (3H) cDNA and DNAs extracted from buffy coat cells and tumors corresponding to sporadic forms of bovine leukosis and some human leukemias and sarcomas.