We have studied prospectively 178 subjects exposed to hepatitis B and 120 haemodialysed patients for the presence of HGs antigen, e antigen and DNA polymerase as well as for anti-HBs and anti-HBc antibodies. The results suggest that the DNA polymerase assay enables us to diagnose hepatitis B earlier than the radioimmunoassay for HGs and that DNA polymerase might be present in the blood in the absence of HBs in cases of confirmed hepatitis B. A posititive correlation between e antigen and DNA polymerase was observed in 83% of the patients on haemodialysis who developed hepatitis B but only in 9% of normal patients developing the same disease.