Peripheral blood T-lymphocyte subsets and cells reacting for Leu 7 antigen, which identifies a subset of killer and natural killer cells, have been examined in 32 patients chronically infected by the hepatitis B or D viruses (HBV, HDV) and in 28 normal subjects. The T8+ lymphocytes were increased and the T4/T8 ratio was decreased in patients with HBV replication (identified by the presence of HBcAg in liver and HBV-DNA in serum) and in patients with HDV infection (HDAg in liver). These patients had more active liver disease than patients without evidence of viral replication, B or D, who showed normal lymphocyte counts. Leu 7+ lymphocytes were also increased in patients with viral replication and active disease and correlated positively with alaninaminotransferase serum levels. These observations suggest the participation of both T8+ and Leu7+ cells in the pathogenesis of liver cell injury in HBsAg-positive chronic liver disease.