The small ACTH activity of the neurointermediate lobe of the porcine pituitary is authentic ACTH (1-39). Porcine neurointermediate lobes were extracted in glacial acetic acid containing 0.1% concentrated HCI, 10 mg% dithiothreitol and 112 mg% 2-mercaptoethanol. After a primary separation by chromatography on Sephadex G-50 (Fine) in 1% acetic acid, a hormonally active material co-chromatographing with ACTH (1-39) was further fractionated and analyzed by several physical methods. By each of these methods, a major region of ACTH activity was located which was indistinguishable from an ACTH (1-39) standard run under the same conditions. Thus, a significant proportion of the ACTH biological and immunological activity in porcine neuro-intermediate lobe is due to authentic ACTH (1-39). Since the adrenal cortex cannot be maintained by the presence of the neurointermediate lobe alone, the functional significance of the ACTH present is obscure.