A novel and convenient assay was used to determine the effect of recombinant Interleukin-2 (IL-2) on the function of the blood-brain barrier (BBB). The assay is based on a variant of the West Nile virus, WN-25, which had lost its neuroinvasiveness but not its neurovirulence. WN-25, when injected intravenously, can cause the death of mice only if the function of the BBB is impaired. Sodium dodecylsulphate (SDS), a component in IL-2 excipient, was found to cause a short term breach in the BBB, enabling the penetration of viruses into the brain. Minimal amounts (30 ng/mouse) can induce a breach of about 10 min, which allows 0.1% of the injected virus to enter the brain. These findings demonstrate the possible use of SDS as a mean for intentional introduction of drugs into the brain, however they also call attention to the danger of using detergents as additives for drugs given intravenously.