With an assay based on the radioimmunological detection of the formation of the C-terminal amide function on a neuropeptide Y-like substrate, amidation enzyme activity with apparent Mr of 56,000 and 38,000 was found in pheochromocytoma extracts. The larger molecular form of amidating enzyme was also expressed and secreted from medullary thyroid carcinoma cells in a dexamethasone-suppressible way. Serum contained high levels of amidating enzyme activity with no difference between normal subjects and patients with pheochromocytomas. However, the majority of the amidating activity in serum was of much larger size, Mr between 80 and 105,000, compared to that released from the endocrine cells. No major difference was found between the molecular forms of amidation enzyme from tissues and from serum either in respect of enzyme kinetics or in respect of requirements for the cofactors copper and ascorbate. The major serum forms of enzyme were relatively independent of exogenous copper; however, they could still be quenched by cobber chelating agents. It is concluded that the molecular weight forms of the amidating enzyme circulating in serum are much larger than the soluble enzyme stored and secreted from most endocrine tissues.