The size and mobilization of the pulmonary vascular pool of lymphocytes was studied in young pigs. Blood lymphocytes, labeled in vitro with a fluorescent dye, were injected either intravenously or into the left heart. Comparable high numbers of lymphocytes were found in the lung after 30 min, demonstrating a specific homing of lymphocytes to the lung vascular bed. When the left lung was perfused with a cell-free medium for as long as 4 h in vitro, a continuous venous release of lymphocytes was found that resulted in about 1.5 x 10(9) lymphocytes and 5.7 x 10(6)/min leukocytes. Peripheral blood lymphocytes were labeled with 51Cr in vitro and injected intravenously. Fifteen minutes or 1 day later the lungs were perfused, and the release of cell-bound radioactivity was measured. The data indicate the existence of a large intravascular pulmonary lymphocyte pool. The sequestration of lymphocytes in the lung in many lymphocyte migration studies is not a removal of effete cells but a physiologic phenomenon.