After detergent treatment the 42 nm. Dane particles from the sera of patients with serum hepatitis separate into an outer coat of Australia antigen (Au-Ag) and an inner spherical 27 nm. diameter component resembling a rhinovirus. Antibody in post-hepatitic serum reacted with the inner component (but not with the outer) to yield aggregates resembling those seen in the liver of patients with serum hepatitis. Antibody to Au-Ag was not detected, before or after the attack of hepatitis. It is suggested that, in an attack of serum hepatitis, Au-Ag antibody develops but is subsequently cleared; antibody against the inner Dane component, however, persists.