The surface of polystyrene plates was studied at different stages of the enzyme immunoassay (EIA) and the passive hemagglutination (PHA) test by the method of scanning electron microscopy in the detection of tick-borne encephalitis (TBE) virus antigen. The study revealed that in the process of EIA larger antigens were washed away from the plate surface. The objects detected on the polystyrene surface were identified as conglomerations of the virions of TBE virus, but whole virions were shown to play no decisive role in EIA. The conclusion was made that, due to some specific features of this method, EIA was more sensitive in reaction with small antigens (individual glycoproteids, their small complexes). And, respectively, the PHA test was more sensitive in reaction with large antigenic complexes (whole virions, their conglomerations, immune complexes).