The prevalence of HDAg in the liver of Chinese patients with chronic hepatitis and hepatocellular carcinoma was determined by using direct immunofluorescence and immunoperoxidase. Six patients (6.3%) out of 95 HBsAg carriers with inflammatory liver disease and neoplasia were found to be HDAg positive. HDAg was shown in the liver of 6 patients (7.6%) among 79 patients with chronic hepatitis. The relative frequency of HDAg in cirrhosis-B, chronic active hepatitis B and chronic persistent hepatitis B was 14.3%, 7.1% and 5.9% respectively. These results suggest that a sizeable number of HBsAg carriers are superinfected with HDV. In view of the large amount of HBV carriers in China, the relative minor but distinct presence of HDV poses an important community health problem.