By cannulating the efferent duct of the caudal mediastinal lymph node in sheep, lymph from the lower respiratory tract was collected under physiological conditions for several days. In 18 such preparations the flow rate varied from 4 to 12 ml/hr between individuals and the lymphocyte count between 4000 and 117,000/mm3. The protein content of the lymph plasma averaged nearly 60% of that of the blood, and this indication of the high permeability of the capillary bed of the lungs was confirmed by measuring the time taken for intravenous doses of 125I-albumin to equilibrate between the blood and mediastinal lymph plasma. The concentration of immunoglobulin A was higher in the mediastinal lymph than in blood serum, while the reverse was true of the concentrations of IgG1, IgG2, and IgM. This evidence for the local production of IgA by the intra thoracic lymphoid tissue was supported by the demonstration by immunoperoxidase techniques of IgA-containing plasma cells in sections cut from the caudal mediastinal nodes, and of IgA-containing immunoblasts in the lymph.