The literature on intestinal protozoa contains a number of references to the ingestion of red blood cells by both amebas and flagellates. By many the ingestion of red blood cells is considered evidence of pathogenic ity; by others the red cells are looked on merely as adventitious food particles. The experiments described in this paper were undertaken in an effort to determine which of these positions is the correct one. For many years one of the principal criteria for distinguishing betweenEndameba histolyticaandEndameba colihas been the presence of red blood cells in the former and their absence in the latter. There is evidence, however, that under certain conditionsEndameba colimay ingest red blood cells.1The endameba of the mouth,E. gingivalis, whichwas once accused of being responsible for pyorrhea alveolaris, apparently lives principally on leukocytes and bacteria, although Smith and Barrett2record the ingestion