Bacteroides sp. grown for 24 hours in the CLG medium releases a considerable quantity of ammonia into the medium. Such cultures containing bacterial cells which are rendered non-multiplying by penicillin are used in our laboratory for the routine propagation of Entamoeba histolytica. Further experiments revealed that the quantity of ammonia in such cultures does not show detectable increases or decreases during the period of propagation of amebae. It is suggested that these levels of ammonia produced during the period of bacterial growth might be one of the factors responsible for the prevention of luxurious growth in cultures of amebae in which Bacteroides is the associate organism.