That an active immunity to yellow fever is produced in monkeys after a simultaneous injection of yellow fever virus and immune serum was noted several years ago by numerous workers, but this method was not considered safe for human vaccination. Following the development of the neurotropic form of virus, which no longer produced “yellow fever” in monkeys, Sawyer, Kitchen, and Lloyd (1) introduced the combined virus and immune serum method of vaccination in human beings. The method consisted of a subcutaneous injection of human immune serum followed after several hours by the injection of “neurotropic” yellow fever virus. The immune serum was standardized by determining the amount of serum per kilogram of body weight which would protect a rhesus monkey completely against the viscerotropic virus, and varied from 0.3 to 0.6 cc. per kilogram. Consequently, the amount necessary for human vaccination was from 21 to 42 cc. for an individual weighing 70 kgm.