The present survey for natriuretic factors in human atrial extract was performed by using in vitro assay for the relaxant effect on the contractility of chick rectum. Three distinct components (alpha, beta and gamma) of a potent relaxant activity were found in the chromatographic regions of the crude extract. As alpha-component of Mr 3,000 daltons, a 28-amino acid peptide has been isolated in a pure state and found to elicit potent diuretic and natriuretic activities as well as vasorelaxant activity, when injected into the assay rats. Accordingly, we proposed a name "alpha-human atrial natriuretic polypeptide (alpha-hANP)" for the peptide. The complete amino acid sequence of the peptide has been established by microsequencing as well as synthesis.