AbstractThe magnitude of the output of lymphocytes from Peyer's patches (PP), the morphology of the lymphocytes and their route of exit from PP has been examined in sheep. An extracorporeal perfusion system was used to selectively label a 3–4 m length of the terminal ileum of lambs at 6–10 weeks of age (the mesenteric lymph nodes had previously been excised from most lambs). This part of the intestine contains about 80% of the total PP tissue in a lamb and most of the lymphoid cells in the perfused tissue were within the PP. During a 10 min labeling period, fluorescein isothiocyanate (FITC) was added to the perfusate to label all the cells in the perfused tissues and [3H]thymidine ([3H] dThd) was added at the same time to label only those cells in the S‐phase of the cell cycle. The unincorporated label was then washed from the perfused tissues, the normal blood circulation was reestablished and the lamb allowed to recover from anesthesia. It was established that the labeling was restricted to the perfused tissue and therefore that any labeled cells subsequently found elsewhere in the animal must have emigrated from the terminal ileum. During the 24 h after perfusion 1.4 × 109−3.9 × 109lymphoid cells (i.e.FITC labeled) left the perfused tissues via the lymph; 12–19% of these cells were either in the process of dividing or less than 24 h old (i.e.[3H] dThd labeled). The majority of the labeled cells probably came from the PP and most were classified as small lymphocytes although the [3H] dThd labeled population included a high proportion of large lymphocyte and lymphoblasts especially during the early hours after the perfusion. The labeled lymphocytes entered the blood in substantial numbers which increased linearly with time so that by 24 h about 7% of the lymphocytes in the blood were fluorescent. The numbers of newly produced cells began to increase rapidly in the blood only during the 12 h to 24 h period. The number of labeled cells in the blood was reduced by about 95% when the lymph from the perfused tissues was drained from the lamb during the experiment. This result provides clear evidence that the vast majority of all of the cells that leave the PP do so via the lymph and not vi