Summary: An extracellular encystment enhancing activity (EEA) greatly stimulates cyst formation by under several different conditions. The activity appears when conditions are suboptimal for growth. EEA does not induce encystment by itself, but enhances differentiation initiated by other factors. EEA titres and their relationship to differentiation are described for encystment induced by: (1) berenil: (2) glucose starvation; and (3) total nutrient starvation. Extracellular EEA was required for maximum encystment by low density cultures in (1) and (2), but not in (3). It was required during the period of visible cyst wall formation rather than for earlier events in encystment. The effectiveness of EEA was reduced under some conditions by spontaneous changes in cellular sensitivity, and by extracellular inhibitors.


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