Antibody to the y determinant of hepatitis B surface antigen (HBsAg) was induced in a chimpanzee chronically infected with hepatitis B virus and circulating HBsAg subtype adw. The chimpanzee was immunized with purified preparations of HBsAg subtypes adw and ayw. Antibody to HBsAg (anti-HBs), specific for the y determinant, appeared 6 wk post-immunization. No change occurred in titers of HBsAg or antibody to hepatitis B core antigen (anti-HBc), and e antigen remained detectable. The circulating HBsAg subtype adw remained present despite the persistance of anti-y for greater than 8 mo.