Integration of bovine leukaemia virus DNA was investigated at all stages of infection in cattle. We report here the detection of integrated proviral DNA in the majority of antibody positive animals without lymphocytosis. In all but one case virus was integrated at a number of different sites. Hybridization analysis failed to detect proviral sequences in animals shown to be BLV antibody-negative by the Agar Gel Immunodiffusion assay. The pattern of integration in leukocytes from animals with persistent lymphocytosis was similar to that in sero-positive animals without lymphocytosis in that multiple sites of integration were evident. As reported by others only one or a few sites of integration were detected in tumours from enzootic bovine leukosis animals. Tumours from different sites in individual animals were either monoclonal or oligoclonal.