Stimulation of encystation of Entamoeba invadens by incubation of trophozoites under glucose-limiting conditions brought about a dramatic fall of ornithine decarboxylase activity, a key enzyme in polyamine biosynthesis. Levels of enzyme specific activity after 24 and 48 h of encystation represented only 11% and 1.3%, respectively, of those detected at the start of incubation. Induction of encystation in the presence of exogenously added polyamines resulted in a marked reduction in cyst formation. Thus, after 72 h of incubation, 1.0 mM putrescine, 1.0 mM spermidine or 0.5 mM spermine reduced encystation by 48 to 56%. Inhibition was enhanced to 70-73% in response to a two-fold increase in the concentration of either putrescine or spermine. Our results indicate that polyamine biosynthesis from ornithine is rapidly turned off at the onset of encystation.