A quantitative solution hybridization assay (Genostics Tm hepatitis B viral DNA, Abbott Laboratories) for hepatitis B viral DNA (HBV DNA) in serum was compared with the classic spot hybridization test in chronic hepatitis B carriers undergoing antiviral therapy. Twenty-eight patients with HBsAg- and HBeAg-positive chronic liver disease were studied and 128 sera were analysed during treatment. Ninety-nine (86%) sera were found to be positive with the two assays. Fourteen sera were negative in both assays. Fifteen samples were positive by Genostics and negative by spot test. Therefore, the sensitivity of the Genostics assay was equivalent to or slightly greater than that of the spot test. Changes in HBV-DNA levels measured by the two assays showed a high correlation during therapy, but changes in the viral replication level seemed to be better resolved by Genostics. The Genostics assay represents a standardized, easy-to-use, quantitative method for the detection of HBV DNA in serum. It will be useful for monitoring antiviral therapy and should now be evaluated on a large scale.