By using the synthetic peptide ACTH1-24 as a model substrate, an enzyme that may be involved in the amino-terminal acetylation of certain proteins and growing nascent polypeptide chains has been found in hen's oviduct. It was partially purified by a four-step procedure comprising extraction from the homogenates, ammonium sulfate fractionation, chromatography on a column of QAE-Sephadex A-50, and gel filtration on a Sepharose 6B column. An enzyme preparation purified about 40-fold from the homogenates transferred the acetyl group from acetyl coenzyme A preferentially to the amino-terminal amino acids of several ACTH-related peptides at an optimum pH of around 7.2. This occurred to different extents depending on the peptide length and on the nature of the amino-terminal residue. The molecular weight of the enzyme was estimated to be approximately 250,000 by gel filtration.