In order to evaluate the potential infectivity of blood of hepatitis B patients, the Dane particle associated DNA polymerase was determined, which is a reliable marker for the presence of complete viral particles. Enzyme activities were compared with hepatitis B e antigen (HBeAg) titers determined by radioimmunoassay. Detectable DNA polymerase activity was only present in HBeAg positive blood, preferentially in samples with high antigen titers (1 : 1000 and above). These samples therefore have to be considered as highly infectious. However, blood with low HBeAg levels and free of detectable polymerase activity can still be infectious, since the polymerase reaction is rather insensitive compared to the radioimmunological HBeAg determination.