Abstract: A method and test kit are disclosed for detecting the presence of hepatitis B virus in a test specimen containing at least a portion of the DNA of the virus. A test reagent comprises cloned hepatitis B virus-DNA that has been repurified by treatment with a restriction enzyme and labelled to high specific activity with a radioactive label. The sample to be tested is fixed to a solid matrix, incubated in the presence of the test reagent under hybridization conditions and detected by hybridization to the labelled DNA probe. The uncombined HBV-DNA (labelled) is removed from the substrate, and the hybridized HBV-DNA determined by scintillation counting or by autoradiography of the substrate.