Monoclonal antibodies (MoAbs) to Central European tick-borne encephalitis virus (strain Hypr) were used for differentiation of eight viruses of the TBE complex by indirect immunofluorescence. MoAb 11/B3 (in Western blot recognizing 52 and 70 kD polypeptides) reacted with five out of the eight TBE complex viruses, MoAb 13/E5 (anti-52 kD protein) reacted with the western or eastern subtype of TBE virus only, while MoAb 12/G4 (anti-70 kD protein) distinguished the western subtype of TBE virus from the rest of the TBE complex. These three MoAbs were able to differentiate the virulent strain Hypr from attenuated strains Skalica and Hy-HK-18-"3". MoAb 2/10C (anti-56 and 70 kD proteins) which reacted with all viruses of the TBE complex, recognized both virulent and attenuated strains of TBE virus.