Résumé : Bovine leukemia virus (BLV) proviral integration and viral RNA expression were studied in target cells (B lymphocytes) of BLV-infected cattle showing different responses to BLV infection. From restriction DNA analysis, it appeared that no proviral sequences were detected in circulating leucocytes of asymptomatic BLV carriers indicating that less than 5% of the total circulating leucocyte populations harbored the provirus. Proviral sequences were found to be integrated at many sites in the genome of circulating leucocytes of animals in persistent lymphocytosis (PL). Proviral genes were integrated either at very few sites or at many sites in the circulating leucocytes of animals at the tumor stage of enzootic bovine leukosis (EBL); in contrast, BLV provirus was always found to be integrated at one site in the DNA of tumor cells. In the vast majority of cases, we were unable to detect BLV RNA expression in lymphoid tumor cells or circulating leucocytes of affected animals. © 1980.