Eighty-two mental patients were inoculated with Plasmodium vivax. Sixtysix of them were infected with sporozoites obtained from Anopheles quadrimaculatus and 16 with parasitized blood. Three individuals were completely resistant to infection after three inoculations each, with suspensions of sporozoites which were infective on other patients inoculated simultaneously. Thc other 79 patients developed blood infectíon. The strain of Plasmodium was obtained by the author from a case of natural infection from Martínez de la Torre, Veracruz, ncar thc Gulf coast.

The degree of immunity was independent of the geographical origin of the patients. Persons from widely separated points showed similar degrees of immunity. The persistence of immunity may be considerable. One immune individual has been a resident of Mexico City, where there is no malaria, for twenty years. Another patient showing considerable resistance to infection has lived in Mexico City for thirteen years. The period of latency in 15 patients showing relapse ranged from 161 to 259 days (mean, 196.0 days). The longest period of parasite activity observed was of approximately 11 months.