Transfer factor activity was demonstrated in the dialysable extract from lymphocytes from outbred donor mice, in which immunization with live attenuated Langat (E5 "14") virus induced a stage of high resistance against challenge with virulent tick-borne encephalitis virus. Administered to naive inbred recipient mice, the extract conveyed in them specific cytolytic activity, exerted by their T lymphocytes, as demonstrated by the 51Cr release assay on tick-borne encephalitis virus-infected syngeneic L-929 cells.