Although considerable research has been directed towards understanding of the etiology of essential hypertension in man, the role of the kidney enzyme renin and the product of its enzymic activity, angiotensin, in this disorder remains to be more fully elucidated. One parameter of the renin-angiotension system which has not been extensively investigated is that of renin substrate (angiotensinogen). A method for the radioimmunoassay of this alpha-globulin is thus proposed, based upon its unique protein structure. By selective cleavage of angiotensinogen, a small molecular weight polypeptide will be obtained; this peptide will be coupled chemically to a polypeptide carrier and utilized as an antigen for specific antibody production in experimental animal. The produced antibody will then be utilized in the conventional methods of radioimmunoassay for detection and quantation of angiotensiogen.