In patients with essential hypertension, an additional plasma protein with an apparent molecular weight of 15,000 has been observed by high resolution sodium dodecyl sulphate polyacrylamide gel electrophoresis in the presence of beta-mercaptoethanol. Gel filtration on Sephacryl S-300 superfine revealed a native molecule with a molecular weight of 105,000. After reduction of the disulfide bridges with beta-mercaptoethanol the native molecule is dissociated into two subunits of 45,000 and one of 15,000. The native plasma protein has been purified by a combination of gel filtration on Sephacryl S-300, affinity chromatography on Concanavalin A Sepharose and anion exchange chromatography on DEAE-Sepharose. This protein has been referred to as "hypertension associated protein" (HAP).