Serum angiotensin conversion enzyme (serum ACE) is a dipeptidylcarboxypeptidase which activates angiotensin I to angiotensin II and inactivates bradykinine. It is a glycoprotein with an MW of 126,000 to 480,000. It is produced by all endothelial cells, and is located on the cell membrane. It is inhibited by EDTA (chelator of Zn-- cofactor), teprotide (snake venom nonapeptide) and captopril. Estimation of ACE has greatly benefitted from the use of synthetic tripeptides. An example is the method of Cushman and Cheung using hippuryl histidyl leucine. A raised serum ACE level in sarcoidosis has been demonstrated by Liebermann in 1975. The diagnostic value is limited by the existence of high levels in other pulmonary diseases (asbestosis, silicosis). Serum ACE levels in sarcoidosis are all higher when the disease is diffuse from a pulmonary and extrapulmonary standpoint. They decrease when the disease regresses spontaneously and rise if it worsens. Radiological improvement in pulmonary sarcoid lesions under the influence of corticosteroid therapy is accompanied by a fall in serum ACE levels. Persistence of this normalization as the dose is decreased is a favourable sign, whilst the reappearance of a high serum level may either reflect simple and isolated biological "rebound" or may accompany a recurrence of signs of the disease. Serum ACE measurement is thus an important factor in the surveillance of cases of treated sarcoidosis when the dose of corticosteroids is to be reduced.