A new antigen-antibody system was recently described in hepatitis B surface antigen (HBsAg)-positive sera. Despite indications of heterogeneity in specificity, the designations "e antigen" and "e antibodies" are used for the system as such in this article. Eight of 17 long-term carriers of HBsAg with a histological picture of chronic persistent hepatitis or chronic aggressive hepatitis carried the e antigen, while none had demonstrable e antibodies in serum. Ten of 12 healthy carriers with e antibodies were blood donors who had donated 95 units of blood; none of these carriers was associated with a reported case of posttransfusion hepatitis. In five individuals in the incubation stage of hepatitis B, e antigen appeared simultaneously with HBsAg but before the rise in transaminase levels. This finding further links e antigen to hepatitis B. (JAMA 231:356-359, 1975)