After cloning the hepatitis B virus genome molecular hybridization techniques have been established for detection of HBV-DNA in serum and liver tissue. HBV-DNA is presently the most sensitive marker of viral replication and infectivity, which was previously related to the presence of HBeAg in serum and HBcAg in liver cells. HBV-DNA is usually analysed by dot blot-, Southern blot- and in situ hybridization techniques. The study of HBV-DNA has become a valuable part of the routine diagnostic in chronic hepatitis B, providing a more reliable estimation of viral replication and contagiousness and better parameters for severity and prognosis of the chronic infection.