Abstract: Hepatitis B virus (HBV) DNA has been detected in the serum of 56 cases by the polymerase chain reaction (PCR) in this experiment and the results are compared with those obtained by serum marker determination. The data have shown that HBV DNA could be found at a rate of 100% in those who have positive reaction for HBsAg, HBeAg and anti-HBcAg. In addition, 45% cases with negative serum marker also have detectable HBV DNA. Compared with other data from serum maker determination, the results indicate that PCR technique is of higber sensitivity for determination of HBV DNA and may be used in future clinical practice.