Interferon assays and quantitative determinations of virus were performed on specimens obtained from a patient dying with an infection with Western equine encephalomyelitis virus and on tissues taken at autopsy. Interferon and virus content were highest in suspensions of tissue from the basal ganglia. The thalamus contained only slightly less interferon but virus content, relative to the basal ganglia, was disproportionately reduced. Suspensions of temporal lobe tissue contained no detectable interferon, although virus quantity was comparable to that in the thalamus. Interferon was demonstrated in cerebrospinal fluid, but no virus was detected. Neither interferon nor virus was found in other portions of the brain, nor in liver, kidney, or spleen tissues. Interferon could not be detected in serum. Maximum interferon levels in brain tissue contrasted sharply with those obtained in patients of the same age and dying at comparable intervals after onset of St. Louis encephalitis (1,600 versus 25 culture-protective units per g of tissue). Results of in vitro lymphocyte culture experiments suggested that findings in these two infections might be best related to the intrinsic capacities of these viruses to induce human interferon.