Spherical 22-nm hepatitis B surface antigen (HBsAg) particles with a subtype adr were purified from plasma of asymptomatic carriers of hepatitis B virus. When purified HBsAg preparation was treated with sodium dodecylsulfate in the absence of reducing agents, it yielded spherical particles with a diameter smaller than 22 nm, and in addition, a polypeptide with a molecular size of 49,000 daltons, which seemed to constitute the outer coat of HBsAg particles. The recovery of the polypeptide on the basis of optical density at 280 nm was 2%, starting from 22-nm HBsAg particles. The 49,000-dalton polypeptide apparently represented a structural unit of the surface of HBsAg particles, since it bore all common (a, Re) and subtypic (d, r) determinants with essentially the same antigenic titers as intact HBsAg particles. Furthermore, this polypeptide was equally immunogenic as 22-nm HBsAg particles in raising corresponding antibodies in mice. When the 49,000-dalton polypeptide was reduced in the presence of 2-mercaptoethanol, it cleaved into 22,000- and 27,000-dalton polypeptides with a drastic decrease in both antigenicity and immunogenicity. These results indicate the different molecular arrangements between outer coat and inner portion of HBsAg particles, and a potential application of the 49,000-dalton polypeptide as a component vaccine, owing to its strong antigenicity both in vitro and in vivo.