In normal rabbits, lymphocytes have been identified in large numbers on the luminal epithelial aspects of both Peyer's patches and the appendix by scanning and transmission electron microscopy. Luminal cells shared an intimate relationship with intestinal microorganisms. Irrigation of the appendiceal lumen has proved a useful technique for collecting 8.5 x 10(6) (mean) viable lymphocytes (73%) and macrophages (22%). Functional characteristics (mitogen responsiveness, T-cell numbers and cells with positive cytoplasmic immunoglobulin A fluorescence) were similar to isolated intestinal mucosal cell preparations. In vivo cytokinetic studies indicated that 50% of luminal appendiceal lymphocytes were recently divided (within 5 h). The passage of lymphoid cells into the the appendix lumen was antigen responsive, as was the size of the appendiceal lymphoid mass. This evidence suggests that migration of lymphoid cells into the lumen of the intestine in healthy animals occurs normally probably from the gut-associated lymphoid tissue--a phenomenon similar to that observed in the bronchial tract. Both mucosal surfaces may, therefore, be important sites of lymphocyte and macrophage traffic.