Saliva of blood-feeding arthropods promotes infection by the vector-borne pathogens they transmit. To investigate this phenomenon in vitro, cultures of mouse L cells were treated with a salivary gland extract (SGE) prepared from feeding ticks and then infected with vesicular stomatitis virus (VSV). At low input doses of VSV, viral yield was increased 100-fold to 10000-fold by 16–23 h post-infection compared with untreated cultures, and depending on the SGE concentration. SGE-mediated acceleration of viral yield corresponded with the earlier appearance of VSV nucleocapsid protein as detected by 2-dimensional electrophoresis of infected cells. The observation that physiological doses of virus (i.e. doses likely to be inoculated by an infected arthropod vector into its vertebrate host during blood-feeding) respond to SGE treatment in vitro provides a new opportunity for identifying the factors in tick saliva that promote virus transmission in vivo.