Among sera from 160 patients with hemophilia B from 9 centers in Europe and North and South America, 2.5% were positive for hepatitis B surface antigen (HBsAg), 60for antibody to HBsAg, and 31% for antibody to the hepatitis B core antigen. Evidence of exposure to the hepatitis B virus appeared to be related to severity of disease and age rather than the source and method of manufacturer of factor IX concentrate.