Abstract
1. The total bacterial floras from invasive strains ofEntamoeba histolytica, when added to non-invasive strains, do not increase the invasiveness of the latter.2. The bacterial flora of non-invasive strains do not inhibit the invasiveness of virulent strains.3. Bacteria from a virulent substrain (revived by serial liver passage) do not increase the invasiveness of the attenuated parent strain.4. It is concluded that the virulence of a strain ofE. histolyticais primarily dependent upon the invasiveness of the amoeba itself.