In situ hybridisation was used to detect RNA of Hepatitis B virus (HBV) in peripheral mononuclear blood cells. Presence of HBV-RNA was detected in 10/17 HBsAg positive HBeAg positive patients, in 8/18 HBsAg positive HBeAg negative patients, in 1/3 anti-HBs positive subjects. Four control subjects were negative. The frequency of labelled cells varied from 1 to 10% of mononuclear cells. These results indicate that the DNA detected so far in mononuclear blood cells may be transcriptionally active.