In recent years, considerable evidence has accumulated which proves that bacteria can be cultivated from the unexposed tissues of the body, especially the lymphatic glands of normal men and the usual laboratory animals. In contrast to these findings, it was formerly assumed that the unexposed tissues of the body are sterile. Absolute freedom of the internal viscera from bacteria would constitute a rather remarkable relationship to our bacterial environment. When we consider the intimate contact of bacteria with skin and more especially the mucous membrane of the alimentary tract, it would be rather unexpected that these structures should form an absolute barrier against the entrance of bacteria. Consequently, it is not surprising to find a fairly definite flora present in the interior of the body under normal conditions.