Hepatitis B core antigen, antibody to core antigen, and DNA polymerase activity were measured in sera from a select group of post-transfusion hepatitis B patients who had been followed prospectively following blood transfusion. Preliminary results of this study have revealed (1) that RIA testing of blood would not eliminate but would reduce posttransfusion hepatitis B infections by about 50 per cent; (2) that infection with HB virus is modified or aborted in the presence of preexisting antibody to HB surface antigen; and (3) that transfusion of blood containing anti-HBSdoes not increase the risk of post-transfusion hepatitis B. HBCAg and/or DNA polymerase activity were observed in the sera of all recipients tested who developed liver enzyme abnormalities along with HBSAg and anti-HBC. DNA polymerase activity usually occurred in the early stages of incubation before the transaminase became abnormal, whereas HBCAg was more often associated with increasing enzymatic evidence of liver damage, suggesting release of core structures from the hepatocytes. The presence of DNA polymerase without detectable HBCAg may be due to the presence of intact Dane particles in the sera, preventing recognition of the core antigen. No serological evidence of hepatitis B was observed in the sera of 24 other recipients who developed abnormal transaminases. Immunoelectron microscopy of these same sera revealed evidence of exposure to hepatitis A antigen following transfusion in at least two recipients.