The present study was designed to test the effect of bacterial endotoxin on penetration of viruses into the central nervous system (CNS). As a model we used two neurovirulent viruses that lack neuroinvasive capacity: West Nile virus-25 (WN-25) and neuroadapted Sindbis virus (SVN). Administration of lipopolysaccharide (LPS, 100 micrograms/mouse) to CD-1 mice, followed by WN-25 inoculation resulted in 83% encephalitis and death, compared with less than 5% in controls. The results in SVN-inoculated CD-1 mice were quite similar. LPS-treated mice suffered 62% mortality compared with 6% in the nontreated group. No changes in viral neuroinvasiveness were demonstrated in viruses isolated from brains of encephalitic mice, suggesting that neuroinvasion is not due to a selection process for an invasive variant, but to direct penetration of the viruses through the blood-brain barrier (BBB). LPS did not induce WN-25 encephalitis in LPS-insensitive C3H/HeJ mice, compared with 100% neuroinvasion in C3H/HeB mice. Induction of neuroinvasion could be transferred to C3H/HeJ mice by transfusion with serum obtained from LPS-treated, LPS-responsive mice. Passive immunization of CD-1 mice with anti-mTNF antibodies before LPS administration did not prevent LPS-induced WN-25 encephalitis. Furthermore, neutralization of tumor necrosis factor activity in the serum of LPS-treated mice did not abolish its activity, and transfusion-associated encephalitis was observed after the administration of the neutralized serum with WN-25. We suggest that LPS can contribute to virus penetration from the blood into the CNS, a process which turns a mild viral infection into a severe lethal encephalitis. This effect is mediated by soluble factors, and is probably achieved by injury to cerebral microvascular endothelium and modulation of BBB permeability.