1.Evidence is presented to show that the yellow fever protection test in mice, as used in epidemiological surveys, is highly specific.
2.Of a total of 876 human sera from Asia and Australia, where yellow fever has presumably never been present, only 2 specimens, or 0.23 per cent, showed protection against yellow fever virus. Among 481 sera from Italy, Spain, Portugal, Canada, and the United States, where yellow fever was formerly present but is now absent, only one was protective.
3.A total of 1177 sera were tested from the following islands of the West Indies: Barbados, Cuba, Jamaica, Puerto Rico, St. Lucia, and Trinidad. Among them were 821 from persons under 20 years of age, none of whom gave blood-serum which was protective against yellow fever virus. Of 356 sera from adults over 20 years old, 30, or 8.42 per cent, showed immunity to yellow fever.
4.A total of 1089 sera were tested from Mexico. There was a large percentage of protective sera from donors of all age-groups except very young children. All sera from children born after the year 1925 gave negative results, suggesting that yellow fever has probably disappeared from Mexico within recent years.
5.A total of 1182 specimens were tested from the seven Central American countries. The results for El Salvador, Guatemala, British Honduras, Honduras, and Nicaragua were similar to those for Mexico. The sera from persons under 20 years of age in Panama and Costa Rica gave no protection against yellow fever virus, while the percentage of immunes in the older persons considered as one group was about the same as in the other Central American countries.