Serum HBV-DNA is considered the best parameter for monitoring HBV replication in the liver. The filter hybridization assay (spot test) with 32P-labelled HBV-DNA has been the technique more frequently used to date. A simple solution hybridization assay, in which 125I HBV-DNA is used as labelled probe, has been recently standardized. We have compared the performances of these two assays for the detection of HBV-DNA. The results were similar with the two methods: an agreement was found in 39/44 (89%) samples. Three sera were positive only by the spot assay-and two only by the liquid phase assay. However, in these cases, HBV-DNA levels were near the sensitivity limits of the assay. Therefore, the filter and the liquid phase assays can be considered to be suitable methods to monitor HBV replication, a fundamental index for the clinical assessment and prognosis of patients with HBsAg positive chronic hepatitis.