Tick-borne encephalitis (TBE) virus was crosslinked by dimethylsuberimidate (DMS) and the cleavable dimetyl 3,3'-dithiobispropionimidate (DTBP). Analysis by SDS-PAGE revealed polymers of the virus core protein V2 and the glyco protein V3 in continuously decreasing amounts. The formation of higher order complexes was not favoured over the formation of lower order complexes. This is consistent with an even distribution of V3 molecules on the surface of the virion and of V2 in the core. The formation of polymers was completely abolished by SDS, whereas crosslinking of TBE virus disrupted with a large excess of mild detergents (Triton X-100, octylglucoside, Na-deoxycholate) still yielded V3 dimers but only negligible amounts of higher polymers. This indicates that in the presence of these detergents the basic subunit of the TBE virus envelope is composed of two V3 molecules, probably associated with V1. Using two-dimensional PAGE analysis of DTBP crosslinked complete virions or cores, no heterocomplexes between different virus proteins could be found which were small enough to penetrate a 5% gel. Crosslinking between V3 molecules only and V2 molecules only was therefore highly favoured over other reactions.