Sera of thirty-five hepatitis B surface antigen (HBsAg) positive hemodialysis patients and fifty-three asymptomatic HBsAg chronic carrier were studied to assess the relationship between markers of virus activity (hepatitis B e antigen and core-associated DNA polymerase activity) and titers of HBsAg and of antibody to hepatitis B core antigen (anti-HBc). All sera were tested by solid-phase radioimmunoassay methods. HBeAg was detected in 20 (51%) hemodialysis patients and in 14 (26%) asymptomatic carriers, whereas DNA P activity was present in sera of 21 (60%) and 17 (32%) respectively. The highest titers of HBsAg and anti-HBc expressed as P/N ratio between positive and negative controls, were detected in the majority of hemodialysis patients, whereas asymptomatic carriers showed low titers of these markers. These data suggest that in HBsAg positive hemodialysis patients a more active viral replication occurs and further underline the difference between these patients and other categories of HBsAg carriers in terms of infectivity.