A new determinant of hepatitis B surface antigen (HBs Ag), g, which is distinct from the previously identified determinants a, d, y, w, and r, was studied. This new antigen tended to be associated with the d antigen; it was found in 97% of ad specimens and in 15% of ay specimens. With few exceptions, the atypical subtypes, adg- and ayg+, were identified in sera from blood donors, but not in sera from patients with acute viral hepatitis; this finding possibly reflects a reduced tendency of these viral strains to evoke a host immune response. The ability to identify g represents a refinement of HBs Ag serotyping that may be of value in the study of the epidemiology of hepatitis B virus.