A protein free autoclaved medium, used in bacteriological work and modified by Warren (1957, Sc. D. Dissertation, Johns Hopkins University) for the growth of S. cruzi does not support growth of T. rangeli. It was also demonstrated that the former organism did not produce the necessary substances for the growth of T. rangeli and that the labile materials of blood required by this flagellate are dialyzable. S. vespertilionis grows very well in the medium; Endotrypanum schaudinni and Leishmania enriettii grow poorly and T. lewisi did not grow at all. Only medium prepared with the addition of horse serum and hematin supports growth equivalent to that obtained with the original formula using whole blood. Vitamin C could not be demonstrated in the liquid medium. On a nutritional basis it is hypothesized that Schizotrypanum is closely related to the primitive flagellates of some insects.