The mouse protection test, using human sera and the Lansing strain of poliomyelitis virus adapted to mice, gives results that are clear-cut and consistent. An appreciable % (68.7 + ) of the human sera tested protected mice against this virus. The individual''s experience with clinically recognized poliomyelitis, or lack of it, did not determine the presence or absence in his blood serum of protective antibodies against the virus employed. The % of sera giving protection increased with the age of the donors, and there was more protection shown by the sera of persons living in orphanages than by those of the same age group living in private urban dwellings. A higher % of poliomyelitis sera tested from Charleston, S. C, gave protection than did those tested from Detroit, Mich.