Introduction Within recent years the attention of various workers has been engaged in the consideration of oxidation-reduction potentials as an explanation for the dependency of Endamoeba histolytica on bacteria in vitro. The possibility that bacteria provide respiratory conditions required by E. histolytica in culture was suggested by failures to obtain growth of amoebae from micro-isolated cysts in sterile filtrates of cultures of bacteria which support the amoebae in vitro (1). It had also been found that cultures of amoebae developed in shorter time from micro-isolated cysts in the presence of bacteria when kept in an atmosphere of 15 percent CO2 than when incubated aerobically (1). Furthermore, the excystation of sterile amoeba cysts had been accomplished in the presence of a reducing agent, cysteine, by Snyder and Meleney (2). In addition to these observations, a considerable literature on oxidation-reduction potentials in bacteriology and protozoology had developed following the exposition of the theory by Clark (3).