Delta antigen is currently thought to reflect superinfection of the liver with a defective RNA virus (delta agent), requiring helper function from hepatitis B virus for its replication. To assess the influence of delta agent on hepatitis B virus replication in patients persistently infected with both viruses and showing chronic liver disease, we measured serum and liver hepatitis B virus DNA in HBsAg-positive chronic liver disease patients who were either positive or negative for delta antigen in the liver. Hepatitis B virus DNA was assayed in the serum of 21 patients with delta antigen-positive/HBsAg-positive chronic liver disease and in 2 1 patients with delta antigennegative/HBsAg-positive chronic liver disease matched for HBeAg/anti-HBe status and underlying liver histology. HBcAg and delta antigen in liver was determined by immunofluorescence or immunoperoxidase staining. In delta antigen-positive/HBsAg-positive chronic liver disease, serum hepatitis B virus DNA was detected transiently in 4 of 21 cases (19%) and was present in these patients at low levels (trace to 2+). In contrast, 9 of 21 (43%) delta antigen-negative/HBsAgpositive chronic liver disease patients were serum hepatitis B virus DNA positive, and five of these had high serum hepatitis B virus DNA levels (3+ to 4+). Serum HBsAg and anti-HBc titers were significantly lower in delta antigen-positive cases and correlated with reduced amount of HBcAg in the liver. There was a reduction in the proportion of delta antigen-positive/HBsAg-positive carriers showing virion or replicating hepatitis B virus DNA forms in the liver (2 of 13), as compared to liver hepatitis B virus DNA in delta antigen-negative/HBsAg-positive carriers (9 of 12). These findings indicate that in delta antigen-positive chronic hepatitis, synthesis of hepatitis B virus genomes and gene products remain suppressed, whereas production of delta components continues at high levels. These observations may be of value in understanding the pathobiology of chronic delta antigen infection and suggest that liver disease activity in delta antigen-positive/ HBsAg-positive carriers is more related to persistent delta infection than to continued hepatitis B virus replication.