Rabbit appendix was studied by correlated scanning and transmission electron microscopy and light microscopy, taking advantage of luminal bacteria as naturally occurring markers for following antigen uptake. The three-dimensional relationship of lymphoid follicles to other structures was revealed clearly. The smooth luminal surface of the appendix is interrupted by stomata each leading to a recessed interval between the dome epithelium covering the apical pole of each follicle and the goblet cell-rich epithelium forming a cupola on the opposing surface. Bacteria abound in this recess. They are taken up by follicle associated epithelial (FAE) cells which comprise part of the dome epithelium. Intraepithelial macrophages phagocytize the bacteria. Bacteria-laden macrophages are numerous in the dome area of the follicle and in the germinal center. Large lymphatic sinuses surrounding the follicles are well visualized following stimulation of fluid movement. Plasma cells are located within the dome epithelial layer and immediately beneath it. Plasma cells may be concentrated along the base of epithelial cells, with the absence of a basal lamina indicating an absence of the molecular selection which it would normally provide. The appendix, like Peyer's patches, takes up antigens, processes them, and gives evidence of reaction to them locally, with the secretions from plasma cells possibly modulating antigen entry.