We previously reported partial purification of a proteinaceous substance with cytotoxic and enterotoxic activity isolated from the soluble fraction of sonicated axenically cultivated Entamoeba histolytica trophozoites. Demonstration of cytotoxic activity of the preparation (amebal toxin) was dependent on removal of serum from the tissue culture assay system. The objective of the present study was to identify the factor(s) in non-immune sera responsible for producing in vitro inhibition of amebal toxin cytotoxicity on HeLa cells. Gel filtration of non-immune sera from adult humans or bovines demonstrated that two portions of the eluate had significant inhibitory activity against the toxin. A high molecular weight inhibitory fraction was identified as predominantly alpha-2 macroglobulin and a low molecular weight inhibitory fraction was identified as predominantly alpha-1 antiprotease. Preparative isoelectric focusing of human serum isolated inhibitory fractions containing these same alpha globulins. Alpha-2 macroglobulin was purified and alpha-1 antiprotease was partially purified from human sera by other methods and shown to have high inhibitory activity against the amebal cytotoxin. Substances that were inhibitory to the cytotoxic activity of the amebal toxin also mediated reattachment of toxin treated HeLa cells. We conclude that the characteristics of the serum inhibitors, especially their ability to reverse the cytotoxic effects of amebal toxin on HeLa cells, suggest that the amebal toxin has protease activity.