DNA hybridization and cell separation techniques were used to determine which blood components contained hepatitis B viral DNA sequences. Free monomer-length hepatitis B virus was found in large amounts in the polymorphonuclear leukocyte cell fraction in two of five HBsAG-positive patients. In these two patients, viral DNA sequences were not detected in the plasma or platelet fraction, whereas the mononuclear cell DNA contained small amounts of a 7.2 kb size unintegrated hepatitis B genome. These studies indicate that the major reservoir of unit-length viral DNA in the asymptomatic hepatitis B carriers studied here was in the polymorphonuclear leukocyte fraction. The basis for the presence of the viral DNA within these cells is presently unknown, but may relate to viral replication within, or phagocytosis of virus by, these cells.