OBJECTIVE To investigate the pathogenicity of tick-borne encephalitis virus (TBEV) to monocytes and its proliferation characteristics. METHODS After being infected with TBEV, the cytopathic effect of monocyte cell line THP-1 was observed and viral titers were evaluated. Cell culture supernatants at different time points after infection were collected and the nucleic acids of TBE virus and the infection percentage were tested by using Real-time RT-PCR and fluorescence-activated cell sorting (FACS); the survival rate of THP-1 after TBEV infection was detected by Dimethylthiazol-carboxymethoxyp- henyl-Sulfophenyl-2H-tetrazolium inner salt (MTS). RESULTS Real-time RT-PCR and cytopathic assay both demonstrated that TBE virus could replicate in monocytes. The virus particles could be detected and visualized by FACS. In addition, monocyte viability was significantly decreased 5 days after infection with TEBV. CONCLUSIONS TBEV can efficiently replicate and proliferate inTHP-1 cells, indicating that monocytes may play an important role in the process of TBEV spreading to various tissues and organs after infection.