A rapid radioimmunoassay (RIA) for the measurement of hepatitis B antigen and antibody is described in which protein A carrying Staphylococcus aureus is used as a solid phase antigamma globulin reagent. The tests are easy to perform, they are completed in two hours, and are easily automatized. The competition principle is used for antigen testing while antibody is measured by direct binding of radiolabelled hepatitis B antigen. Using human reconvalescent antisera, the staphylococcal test for antigen in 25 μl serum has about the same sensitivity as the Ausria test. The staphylococcal test for antibody has at least the sensitivity of the passive hemagglutination test. Both tests are highly specific.