A procedure has been developed for the effective induction of axenic encystation of Entamoeba invadens strain IP-1. Low concentrations of glucose in Diamond's axenic growth medium cause stimulation of the differentiative process. The time course of encystation depended on the density of inoculum. While a culture with an inoculum of 5 X 10(4) cells/ml required about 3 days to initiate encystment, a culture with an inoculum of 1 X 10(6) cells/ml required only 8 hr. Cyst yield was optimal (70%) when a density of 5 X 10(5) cells/ml was employed for the inoculum. Under above conditions it was found that serum is absolutely required while vitamins are not. It was found that encystation could be triggered without change in the osmolarity of the medium and that oxygen does not influence this process.