Effect of certain flaviviruses on the activity of mouse natural killer (NK) cells was investigated using the classical mouse splenocyte system and YAC-1 cells for demonstration of NK cell cytotoxicity. Infection of mice with Langat and West Nile (WN) viruses was accompanied by temporary activation of NK cells. In mice infected with tick-borne encephalitis (TBE) virus the stimulation phase of NK cell cytotoxicity on days 2-4 post-infection (pi) was followed by suppression of their activity. As to the surface markers (sensitivity to antitheta and antiimmunoglobulin serum, respectively), the flavivirus-activated NK cells did not differ from the endogenous NK cells of intact mice. The stimulatory effect of flaviviruses on cytotoxicity of NK cells varied in different mouse lines. An increased NK cell activity at early stages of TBE virus infection was observed in mouse lines characterized by low (C57B1/6) and medium (BALB/c)--but not by high (CBA)-activity of their non-stimulated NK cells. Suppression of NK cell activity at later stages of TBE virus infection was not associated with virus multiplication in mouse splenocytes.