The sensitivity of vesicular stomatitis (VS) viruses to interferon (IFN)-mediated anti-viral effects has been well documented. Previous studies in our laboratory have shown the ability of mosquito saliva to enhance vesicular stomatitis New Jersey (VSNJ) virus infection in mice. To investigate the effect of mosquito saliva on virus replication and IFN α/β expression, virus titers were analyzed at various time points after infection in cells that were treated with mosquito salivary gland homogenate (SGH). Salivary gland treatment of mouse fibroblast cells (L929) resulted in a significant increase in virus growth kinetics compared with untreated controls. In contrast, Vero cells, which are deficient in the IFN α/β response, did not yield increased viral titers in the time points examined. Treatment of L929 cells with an IFN α/β neutralizing antibody also slightly increased virus yield. Ribonuclease protection assays revealed that induction of IFN α2 expression was reduced in L929 cells treated with SGH. Modulation of IFN α/β by mosquito saliva may be a critical determinant of the transmission and pathogenesis of VSNJ virus.