Hog renal renin was measured by a radioimmunoassay of angiotensin I generated during the incubation of the enzyme with an excess of angiotensinogen. The fixation of renin on a sepharose-hexamethylenediamine-pepstatin gel has been studied: the saturation of a 1 ml column of gel is obtained with 900 Goldblatt units of renin. A 70 ml column processing several grams of protein has been used to purify renin by affinity chromatography: a 100 fold purification of renin is achieved in one step. Renin, before and after affinity chromatography, has been analysed by electrofocusing with a pH gradient from 3 to 10; the isoelectric points obtained are respectively 4.8 and 5.0.