The structure and properties of infectious and formaldehyde-treated particles of tick-borne encephalitis virus, concentrated and purified by chromatography on macroporous glass, were studied. In addition to complete virions, such preparations contain some incomplete forms that differ in density, morphology and protein composition (incomplete forms do not contain nucleocapsid protein). The physico-chemical analysis of complete virions showed that formaldehyde treatment causes a) the formation of glycoprotein dimers and b) a portion of nucleocapsid protein to become tightly cross-linked with viral RNA. Formaldehyde treatment of incomplete forms resulted only in the formation of a small amount of glycoprotein dimers. Incomplete forms and glycoprotein extracted from inactivated preparations had protective and antigenic activity.