Standard methods of virus DNA detection using the polymerase chain reaction (PCR) can be time consuming and involve multiple steps during which contamination with exogenous DNA often occurs. Therefore, we developed a simplified method for detecting hepatitis B virus (HBV) DNA in serum. The main advantages of this method are that it can be performed rapidly, it consists of only a few steps, and has a false positive rate of <0.1% in our laboratory. In testing serum from 84 human patients, we found HBV DNA in all patients who had HBsAg and hepatitis B e antigen (HBeAg) in serum and in 64% of the patients who had HBsAg, but not HBeAg, in serum. Also, 3 of 11 patients who were chronic HBV carriers and who subsequently lost HBsAg were found to have HBV DNA in serum. In contrast, all patients who lost HBsAg after acute HBV infection or those classified as having non-A, non-B hepatitis were negative for HBV DNA. Thus, the modified PCR technique is a sensitive and rapid method for detecting HBV DNAcontaining virions in serum.