Abstract: The formation of antibodies to the A-2 plaque virus previously propagated in tissue culture is described. The application describes an immune response against those antibodies and against the A-2 plaque virus antigen which is associated with human viral hepatitis both of type A and B origin. The reaction of antibody for A-2 plaque virus with the antigen from stool specimens of volunteers infected with type A hepatitis and from the sera of patients diagnosed as having type B hepatitis is described. Convalescent sera from both type A and type B hepatitis patients in reaction with the virus, demonstrate the presence of antigenic components in the plaque virus which react with human antibody formed in response to infection with either type A or type B hepatitis. Moreover, the A-2 plaque antibodies react with hepatitis B core antigen, while antibodies to HB.sub.c Ag react with A-2 plaque antigen.