Because of the persistence of serum and/or urinary calcitonin in patients with thyroidectomies and the presence of high levels of immunoreactive calcitonin in the pulmonary tissue of primates with intact thyroids and primates on whom thyroidectomies were done, an immunocytochemical study was made of the lungs of human neonates. With the use of a specific antibody to calcitonin and the application of immunoperoxidase staining, reactivity was located within the bronchial and bronchiolar Kulchitsky (K) cells. This study, which strongly suggests that these cells have an endocrine role, offers a pathophysiologic rationale for the high calcitonin levels associated with the carcinoid tumor and small cell carcinoma, which may originate from the K cell. In addition, this study may explain why the removal of the thyroid gland is not followed by a profound change in calcium metabolism.