Since recovery from an infection with Plasmodium vivax is associated with the development of an acquired immunity to this parasite, it appeared desirable to ascertain: (1) whether this acquired property is a characteristic of long or short duration, i.e., of a few months only or lasting for several years; and, (2) whether this property endures in the absence of a demonstrable latent infection. With the purpose of securing data bearing upon these points two series of similar experiments were performed. As test cases there were selected patients who had spontaneously recovered from an induced infection with the McCoy strain of P. vivax, and who, a varying period after recovery, had been given an intensive treatment with quinine after the method of Sinton. In the second series of experiments each test patient was paired off with another patient of the same inoculation series who had recovered spontaneously but had not been subsequently treated.