By the use of monoclonal antibodies we have recently defined eight distinct epitopes on the structural glycoprotein of tick-borne encephalitis (TBE) virus which differ with respect to location, function, or serological specificity (Heinz et al, Virology 126, 525-537, 1983). The present investigation reveals a complex network of interactions between antibodies directed against distinct nonoverlapping epitopes leading to enhanced binding of certain antibodies in the presence of bound second antibodies. The enhancement between antibody pairs can be either unidirectional or bidirectional. In addition, there are domains of predilection, which bind enhanceable antibodies (domain A) whereas others bind antibodies which preferentially induce enhancement (domain B). These domains represent structurally unrelated entities, domain A being sensitive to denaturation and fragmentation and domain B being resistant. Quantitative evaluation of binding data by Scatchard analysis revealed that the observed enhancement phenomenon is due to a two- to sixfold increase of antibody avidity. In the system described, enhancement of antibody binding is not dependent on antibody bivalency since it could also be demonstrated with purified Fab fragments acting either as enhanced or as enhancing antibody. It is therefore concluded that binding of antibodies to certain epitopes on the TBE virus glycoprotein induces conformational changes in distant parts of the molecule which can result in increased avidity of antibodies directed to conformationally changed epitopes. A possible explanation for the origin of this enhancement phenomenon is presented.