Serum hepatitis B virus DNA is the best marker for monitoring hepatitis B virus replication in the liver. Data are presented on the sensitivity, specificity and application of a standardized solution hybridization assay for the quantitation of hepatitis B viral DNA in serum. The assay is linear over several orders of magnitude in sample concentration and is equivalent in sensitivity and specificity to classic filter hybridization methods employing [32P]DNA probes and autoradiography. The assay provides clinically relevant information for monitoring viral replication before, during and after antiviral therapy.