Sera and liver biopsies from 30 Italian patients, carriers of HBsAg for at least 3 years, were examined for markers of hepatitis B virus (HBV) infection by serological assays and immunofluorescence. Biopsies were analyzed for HBcAg, HBsAg, and δ antigen by immunofluorescence; sera were assayed for HBsAg/anti-HBs, HBcAg/anti-HBc, HBeAg/anti-HBe, δ/anti-δ, HBV-specific DNA polymerase activity and the presence of HBV DNA. HBcAg, HBeAg, and DNA polymerase tests were positive in the sera of 71, 86, and 57%, respectively, of carriers with intrahepatic HBcAg. HBV DNA was detected in 100% of patients expressing HBcAg in the liver with a strong correlation between the concentration of serum DNA and the intensity of HBcAg immunofluorescence in the liver. HBV DNA was detected in the sera of 63% of carriers with intrahepatic δ where the other markers of HBV replication (HBeAg, DNA polymerase) were undetectable. The assay for serum HBV DNA appears to be an excellent noninvasive method for detecting active replication of HBV in HBsAg carriers.