The fate of lymphocytes in the bronchoalveolar tract was studied in normal young pigs. Peripheral blood lymphocytes were labeled with 51chromium or with fluorescein isothiocyanate (FITC) and instilled into a single segmental bronchus. In the first series of experiments, the radioactivity was determined in several different parts of the lungs, individual bronchial lymph nodes and a series of other lymphoid and nonlymphoid organs. About two-thirds of the radioactivity was still in the lung at 1 d. The draining lymph node also contained high amounts of radioactivity. To exclude that this was caused by labeled cell debris, an immunohistologic technique was used to localize the FITC-labeled cells. There were clearly labeled lymphocytes in the sinusoids of the draining nodes. These lymphocytes were characterized by additional surface staining. B, T, TH, and TS lymphocytes were seen much less often than in the inoculum while the null and gamma delta T cells showed a preference. The lymphocytes reached the bronchial lymph nodes via lymphatics. Lymphocytes in the bronchoalveolar space are not effete cells to be destroyed but return to the immune system. These data provide new interest in the immunologic role of lymphocytes in the bronchoalveolar tract and their kinetics.