HBsAg binds to a solid-phase adsorbent consisting of polymerized human serum albumin (HSA) on glass particles. Both AD and AY antigenic subtypes of hepatitis B surface antigen (HBsAg) display this interaction. In either case, the binding to polymerized HSA is reduced in the presence of human serum, suggesting significant attachment of serum components at the locations on HBsAg particles where polymerized HSA binds. The temperature dependence of the interaction goes through a maximum above room temperature, in contrast to the increasing reaction with temperature of the HBsAg--anti-HBs antibody system. The interaction between HBsAg and polymerized HSA is discussed in relation to previous findings of HSA polymers and anti-polymerized albumin antibodies in hepatic patients. A mechanism for production of an autoimmune, antialbumin antibody condition, in association with hepatitis B virus infection is proposed.