Abstract The non-pathogenic trypanosomes, the members of which infect various rodents and are morphologically similar to Trypanosoma lewisi of the rat, present a number of immunological problems of peculiar interest. As far as these infections have been studied, they are characterized by an initial period of rapid reproduction of the parasites followed by a period during which reproduction is either completely inhibited or greatly reduced, and they are generally terminated by self-cure with a subsequent immunity of variable length. This peculiar development, which has been studied extensively in T. lewisi, is associated with the formation of the two following antibodies (Taliaferro, 1932): (1) The inhibition of reproduction is associated with a passively transferable antibody, termed ablastin, which has the specific property of inhibiting reproduction of the organisms and possesses many of the classical features of ordinary antibodies, but differs from them in that it exhibits no marked in vitro affinity for the organisms.