In spite of extensive studies of the pathogenic activities of Entamoeba histolytica upon animal hosts by many investigators, the factors that govern the pathogenesis of the parasite are still largely unknown. Attempts have been made to determine the factors concerned in the pathogenicity of certain strains of E. histolytica from the aspect of parasite itself and from the point of view of its interaction with the host. The enzyme content of the trophozoites of the amoebae examined has been studied in detail, both in the living organism and in its extracts. The total patterns of proteolytic enzymes and of certain other enzymes were determined to establish whether it was possible to define the invasiveness of the various strains of E. histolytica already established as pathogenic by other workers or in our laboratories. It was found impossible to distinguish clearly between the so-called "pathogenic" and "non-pathogenic" strains of E. histolytica by such means.