Federal Republic of Germany. HBsAg's from all sera appear to contain a groupspecific determinant a as well as d or y and w or r type-specific determinants (1, 15). Antigenic variation of some of these determinants and additional determinants such as x and q have also been described (4, 5). Several polypeptides isolated from purified preparations of 22-nm and filamentous HBsAg particles have been shown to contain the HBsAg specificities (7, 10, 12, 22). Recently it has become apparent, however, that HBV is not a unique virus. Viruses with similar ultrastructural, antigenic, molecular, and biological features have been found in eastern woodchucks (25), Beechey ground squirrels in northem California (16), and domestic ducks in China (J. Summers, personal communication). These viruses provide important animal models of HBV infection in humans, and it is of interest to determine how closely related they are. Although the surface antigens of woodchuck hepatitis virus (29) and ground squirrel hepatitis virus (GSHsAg) (16) clearly cross-react in serological tests with HBsAg, it is not known how 787