We have been pursuing the identification of a peptide that was inadvertently detected using antisera directed to the C-terminal domains of prosomatostatin (ProS), the precursor of somatostatin-14 (S-14) and somatostatin-28 (S-28), which are neuropeptides found predominantly in brain and gastro-entero-pancreatic tissues. They both inhibit the release of several hormones and neurotransmitters. Using an antiserum (F-4) reacting with S-28 as an immunoadsorbent to separate it from other ProS-related peptides, combined with a C-terminal-directed radioimmunoassay, we were able to quantify levels of S-28 in plasma and tissues of humans, monkeys and rats. However, tenfold increases beyond S-28 levels were obtained when direct measurements were made in mammalian plasma. On filtration of immunoadsorbed plasma on a sizing gel, we found an F-4 reactive peak antecedent to S-28. Levels of this peak increased two- to threefold after ingestion of food. After tissue extraction in monkeys and rats, this putative peptide was widely distributed in tissues but highest concentrations were found in the distal ileum. By immunocytochemistry, neurons within the myenteric plexus and ramifying into the mucosa were detected in the ileum; however, they did not stain with antisera reacting with the C-terminal S-14 region of ProS. The peptide was isolated from monkey ileum by acid extraction, affinity chromatography and HPLC. The molecular weight was determined to be 1419 daltons and its amino acid composition to be that of the C-terminal 13 amino acid residues of S-28 (S-13). This differs from previous reports that this peptide comprised 12 amino acids. Because we could not identify any S-14 in these cells, we believe that S-13 is either the C-terminal product of ProS, which may have a specific biologic function, or a cleavage product of a hitherto unknown alternative C-terminal gene product of ProS. We are currently evaluating these possibilities.