In the present study we have investigated 53 patients with a spectrum of acute and chronic hepatitis B virus (HBV) infection for the presence of leucocyte HBV-DNA with the aid of molecular techniques. HBV-DNA was detected in peripheral blood mononuclear cells of 31 of 45 (69%) of chronic HBsAg carriers and 2 of 8 (25%) patients with acute hepatitis B. Although HBV-DNA was detected more frequently in leucocytes from those HBsAg carriers seropositive for HBeAg (79%), 50% of those with anti-HBe in serum had leucocytes positive for HBV-DNA independent of the presence of serum HBV-DNA. Examination of various leucocyte subpopulations showed the presence of HBV-DNA in polymorphonuclear leucocytes as well as T- and non-T-enriched mononuclear cell fractions. The HBV-DNA identified was predominantly 3.2-kilobase (kb), while higher molecular weight sequences were rarely detected, and lower molecular weight sequences indicative of active viral replication were not observed. These data indicate that although leucocytes do not actively support viral replication, they frequently harbour 3.2-kb HBV-DNA and may act as a reservoir for infection and, more importantly, since leucocytes contaminate several body secretions, may be involved in virus transmission.