Investigating the interrelationship between Treponema pallidum and the host cells, the authors have studied under the electron microscope ultrathin sections of cellular elements of rabbit chancres and the condition of the treponemes. A description is given of the structure of the basic cellular elements found in the chancres, particularly the plasma cells since it is in them that the main immunological processes take place.The morphology of the treponemes varies in the course of the pathological process as well as with the type of cell harbouring them. Enveloped in multi-layered membranes, the treponemes may lie within plasma cells and may retain all their characteristic features, indicating that they remain viable and that their envelopes can protect them from the action of antibodies and drugs. In long-standing chancres cyst-like formations with multi-layered membranes have been found.