Thesis (Ph.D.)—Boston University / Endamoeba histolytica, the etiological agent of amoebiasis, was first cultivated in the presence of associated bacteria by Boeck and Drbohlav (Am. J. Hyg., 5: 371-407, 1925). Since that time numerous workers have attempted to culture the amebas free of bacteria but have been unsuccessful. The majority of studies on the cultural requirements of this protozoan has been complicated by the presence of more than one species of bacteria. Therefore, the nutritional studies on E. histolytica have been very meager. A pure culture of this organism is essential to the elucidation of its metabolism, physiology, and immunology. It was the purpose of this investigation to identify growth factors required by the amebas so that a pure culture may become available. In order to control the associated bacteria during the assay of growth factors, penicillin and streptomycin were added to the culture medium in high concentrations. This concentration of the antibiotics was shown to sterilize the bacterial flora yet have no detrimental effect on the amebas. Many compounds were tested for their ameba-growth-promoting activity under bacteria free conditions. The test medium consisted of egg slants over-layed with normal horse serum in Ringer solution, dextrose, sodium thioglycollate, rice starch, antibiotics and a vaspar anaerobic seal [TRUNCATED]