Abstract. Hepatitis B virus (HBV) DNA sequences have been detected in leucocytes from HBV-infected individuals. The aim of this study was to assess the specificity of HBV for a special leucocyte subset in nine healthy chronic HBV carriers, nine HBs antigen-positive patients with chronic active hepatitis, 16 HBs antigen-negative haemophiliacs with HBc and/or HBs antibodies, and 10 patients with HBV-related systemic necrotizing vasculitis. HBV-DNA sequences were found by Southern blot hybridization in the leucocytes of 15 out of the 44 (34%) patients. The prevalence was not significantly different between the four groups. HBV-DNA was found in the CD4+ cells (9/11) as well as in the CD8+ cells (4/11), B cells (4/12) and monocytes (2/12). In conclusion, leucocytes, and particularly CD4+ lymphocytes, are frequently infected by HBV in patients with HBV serum markers.