By adsorbing purified Tick-borne encephalitis (TBE) virus antigen onto beads of polystyrene, a solid-phase radioimmunoassay (RIA) for specific antibodies of the IgM- and IgG-class in untreated and nonfractionated human sera could be developed. The antibodies were measured by antihuman-gamma and antihuman-mu globulin labelled with 125 I. The RIA was about 20 times as sensitive as the hemagglutination-inhibition test. About 10% more cases of TBE could be diagnosed with the RIA than with the 2-mercaptoethanol test.