Anantin, a naturally occurring peptide from Streptomyces coerulescens, binds competitively to the receptor of atrial natriuretic factor (ANF) from bovine adrenal cortex (Kd = 0.6 microM) and acts as ANF antagonist. Protein chemical data and FAB-MS have identified anantin to be a cyclic polypeptide consisting of 17 common L-amino acids. The molecule is highly stable and precludes the application of standard sequencing methods. The primary sequence of anantin was determined by 2D 1H NMR spectroscopy and the application of advanced protein chemical methods to be Gly1-Phe2-Ile3-Gly4-Trp5-Gly6-Asn7-Asp8 -Ile9-Phe10-Gly11-His12-Tyr13-Ser14+ ++- Gly15-Asp16-Phe17. The molecule is cyclized between the beta-carboxyl group of Asp8 and the amino group of Gly1.