Alpha-human atrial natriuretic peptide (alpha-h-ANP) was purified to homogeneity from human plasma of healthy adults in a three-step procedure including immunoaffinity chromatography on immobilized monoclonal anti-alpha-h-ANP antibody (moab). A single peak of immunoreactivity was obtained after final reversed-phase HPLC and the amino acid sequence of the isolated material was identical to that of synthetic alpha-h-ANP. No further atrial peptides could be detected although the moab reacts with all biologically active peptides. It is therefore concluded that alpha-h-ANP is the only form of ANP circulating in human plasma.