An assay for DNA polymerase reaction products using slab gel electrophoresis and autoradiography was compared with the spot hybridization technique for the detection of hepatitis B virus DNA in 317 blood samples. The former could identify the nature and size of DNA on electrophoresis, and reduce potentially false-positive results due to artifacts. Discordant results between the two methods occurred in 36 of 317 samples; 22 were positive by the spot technique alone, and 14 were positive by the analysis of DNA polymerase reaction products alone. However, the samples positive with the spot test alone showed weak radioactive signals on electrophoresis/autoradiography that were often interpreted as "inconclusive" by blind observations. Correlation of hepatitis B e antigen/antibody with hepatitis B virus DNA was studied in 91 patients with various chronic liver diseases. Discordant results, i.e., presence of the DNA in antibody positive sera, or its absence in the antigen positive sera, were obtained in 15 (19%) cases. Such patients tended to have advanced liver disease with fluctuating serum aminotransferase levels. Analysis of DNA polymerase reaction products by slab gel electrophoresis and autoradiography is not only sensitive, but is also as specific as the Southern blot technique in the detection of hepatitis B virus DNA in serum, and may prove useful in selected samples, especially where no cloned hepatitis B virus DNA is available, or in search of new hepatitis B virus-like viruses.