So far as is known, viruses appear to possess no enzyme activity ; the virus particles, in order to multiply, must presumably utilize the enzyme-substrate reactions of the host cells, or else have a selective affinity for the products of host metabolism. It is thus possible that an investigation of the biochemical activities of tissues containing a growing virus might disclose an abnormality of enzyme action which would yield information upon the metabolic activities of the virus itself. With this object in view, preliminary studies have been carried out upon the xanthine oxidase content of the brains of mice with yellow fever encephalitis.