Hepatitis B core antigen (HBcAg) particles, approximately 27-28 nm in diameter and rho = 1.30-1.35 g/cm3, were purified from the liver of a chimpanzee experimentally infected with hepatitis B virus (HBV) while under cyclophosphamide treatment. The purified HBcAg particles incorporated radioactive deoxythymidine triphosphate. The product was precipitable by trichloroacetic acid and sensitive to DNase, but resistant to digestion by RNase. The reaction required four deoxyribonucleosise triphosphates- dATP, dCTP, dGTP and dTTP. Exogenous template did not enhance the reaction. From these findings, it was suggested that HBcAg particles purified from the HBV-infected chimpanzee liver contained DNA polymerase and endogenous DNA.