Angiotensinogen (renin substrate) was purified from plasma of nephrectomized rats by a four step procedure using ammonium sulfate fractionation, chromatography on Blue Sepharose CL-6B and SP-Sephadex C-50, and gel filtration on Sephadex G-150. 2. The final preparation had a specific concentration of 9.3 microgram angiotensin I/mg (mean of six separate runs). The best preparation so far obtained contains 14.6 microgram angiotensin I/mg protein, which represents a purity of 62%. 3. By sodium dodecyl sulfate disc electrophoresis an apparent molecular weight of 56,400, and by isoelectric focusing an isoelectric point of 4.85 has been determined. These properties of rat angiotensinogen are similar to those reported for human angiotensinogen.
