A heat-labile material with cytotoxic and enterotoxic activities was isolated from axenically cultivated Entamoeba histolytica. The cytotoxin-enterotoxin was partially purified from the particulate-free supernatant of sonicated trophozoites by ammonium sulfate precipitation and gel filtration. Cytotoxic activity, assayed on monolayers of HeLa or BHK-21 cells, was restricted to proteins that eluted in the molecular weight range of 25,000--35,000 daltons. Cytotoxicity was demonstrated at protein concentrations as low as 2--4 microgram/ml, was heat-labile (75 C, 30 min), and was inhibited by specific immune IgG and by an undetermined factor in nonimmune serum. Enterotoxic activity of the partially purified toxin was demonstrated by induction of fluid secretion in ligated rabbit ileal loops. The cytotoxin-enterotoxin of E. histolytica may play an important role in the production of diarrhea and mucosal injury in amoebic colitis.