The presence of Hepatitis B virus (HBV) DNA in sera of patients with HBV related diseases is considered a reliable marker of virus active replication. In this paper HBV DNA was assayed by the molecular hybridization method with a 32P labeled nick translated HBV probe. The assay was positive in sera of 21 out of 22 HBeAg-positive and 4 out of 8 HBeAg-negative asymptomatic HBsAg carriers. 15 HBsAg-negative sera obtained from healthy donors showed no HBV DNA. Almost 80 per cent of HBeAg and HBV DNA positive sera revealed a DNA-polymerase activity. In order to determine the infectivity of HBsAg carriers, it appears that, whenever possible, the HBV DNA spot hybridization should be performed in conjunction with the DNA-polymerase, HBsAg and HBeAg tests.