The pathogenesis of amoebic dysentery is a result of cytolysis of the colonic mucosa by the parasitic protozoan Entamoeba histolytica. The cytolysis results in extensive local ulceration and allows the amoeba to penetrate and metastasize to distant sites. Factors involved in this process were defined with three clones that express hemolytic activities in Escherichia coli. These potential amoebic virulence determinants were also toxic to human colonic epithelial cells, the primary cellular targets in amoebal invasion of the large intestine. The coding sequences for the hemolysins were close to each other on a 2.6-kilobase segment of a 25-kilobase extrachromosomal DNA element. The structural genes for the hemolysins were within inverted repeats that encode ribosomal RNAs.