An HBsAg negative blood donor, and three of her recipients, who developed HBsAg positive post-transfusion hepatitis B, were all positive for serum HBV DNA by polymerase chain reaction (PCR), and by subtype discriminating PCR were found to harbour HBV specifying ayw. Thus HBV specifying ayw. Thus HBV DNA may be detected and sub-typed by PCR in infectious HBsAg negative individuals.