Proteolytic digestion of purified whole tick-borne encephalitis virus or its isolated envelope glycoprotein (E) in the form of rosettes yields an Mr 9000 fragment that is resistant to further digestion and carries polyclonal and monoclonal antibody-defined antigenic determinants. In a denaturation/renaturation experiment it was demonstrated that the antigenic reactivity of this domain, which was lost upon reduction and carboxymethylation, could be regained if the reducing agent was dialysed out before carboxymethylation. By the use of [35S]cysteine-labelled E protein and amino acid analysis it was confirmed that the reacquisition of antigenic reactivity in the renaturation experiment was associated with the reformation of disulphide bridges, which apparently confer structural stability to this part of the molecule. By the experiments performed we have identified an independently folding antigenically active domain of the E protein that is stabilized by disulphide bridges and has a strong tendency for renaturation.