- To determine whether peripheral blood mononuclear cells (PBMCs) contain replicating forms of hepatitis B virus (HBV) DNA and to define which cell subset may be permissive for viral replication, we analyzed the PBMC DNA from 14 carriers positive for hepatitis B surface antigen (HBsAg) by Southern blot hybridization. HBV-related DNA, which was present exclusively in an extrachromosomal state, was found in the PBMCs of all five hepatitis B e antigen (HBeAg)-positive and three of nine HBeAg-negative carriers. Serum-associated HBV DNA was detected only in those HBsAg carriers whose PBMCs contained HBV DNA forms resembling replicative intermediates (1.0–3.2 kilobase pairs in the EcoRI digests). Analysis of PBMC subsets revealed that replicating forms of the HBV genome were present primarily in monocytes. Low levels of hybridization also were detected in B cells, whereas the T cell fraction (which contained natural killer cells) appeared to be devoid of these replicating forms.