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1. Highly specific antibodies to human renin were prepared in rabbits and used for the preparation of a renin-free substrate, the direct radioimmunoassay of renin in plasma and kidneys, and the localization of renin with fluoresceinated antibodies. 2. In a patient with a partially infarcted kidney, plasma renin activity was increased, and could be activated by acid. The direct measurement of plasma renin by radioimmunoassay gave identical values before and after acidification. 3. In the ischaemic part of the kidney, renin content was high, both by the enzymatic and the direct method of measurement. It was low in the non-ischaemic part of the kidney. 4. All afferent and some interlobular arteries of the obsolescent glomeruli were stained with fluoresceinated anti-renin antibodies. In the non-ischaemic area, the juxtaglomerular appratus did not stain. 5. Renin can now be measured in human plasma and kidney as an entity, by a specific radioimmunoassay. A direct investigation of this intrarenal hormone is now possible at the renal tissue level.