Abstract. A radioimmunoassay method for the measurement of angiotensin II in human plasma is described. The hormone is extracted quickly by Dowex (H+) particles. Angiotensinase and converting enzyme are inhibited by a mixture of EDTA and o‐phenanthroline. Blood samples can be processed at room temperature. Recoveries from blood were 83%± 11 (SD), n=44. Replicates within one assay varied about their mean with a standard deviation of ± 10%, n=16. The smallest concentration detectable, using 10 ml plasma samples, was 2 pg/ml plasma. Peripheral venous plasma concentrations in 33 normal adults ranged from 5 to 35 pg/ml. Plasma angiotensin II concentrations in some physiological and clinical conditions are reported.

Experiments were designed to test the possibility that glomeruli can convert angiotensin I (AI) to angiotensin II (AII) (Thurau, Dahlheim & Granger, 1969). In six experiments, 10\g=m\g(1-asp-5-ile) AI in 1\m=.\1 ml 0\m=.\15m-phosphate\p=m-\salinebuffer, pH 5\m=.\7, were incubated in stoppered neoprene tubes with 20-25 glomeruli dissected as previously described (Brown, Davies, Lever, Parker & Robertson, 1965) from kidneys of New Zealand White rabbits. The glomeruli were stored frozen, then thawed and incubated. In 2 control experiments AI was incubated with the paper fragments used to transfer glomeruli to the incubation tube, in 1 control experiment with 25 boiled glomeruli, and in 4 control experiments with rabbit blood (0\m=.\1\g=m\l )equal in volume to 25 glomeruli (Vimtrup, 1928). In another control experiment glomeruli were incubated without angiotensin. Ten \g=m\g AI were incubated with buffer alone with each set of experiments.An incubation pH of 5\m=.\7was chosen because single glomeruli show little angiotensinase activity at this pH (Brown et al. 1965). Microbial contamination (Vane, 1971) was prevented by making buffers from boiled salt solutions and sterile distilled water. Tubes, stoppers and dishes for microdissection were boiled for 20 min or heated in an oven overnight before use. The incubation buffer contained neomycin (0-2%).The tubes were incubated at 37°C for 48 h. Any AH produced was separated from AI by a modification of the method of Walaszek, Bunag & Huggins (1962). 0-5 ml incubation mixture was diluted with 1 ml boiled, cooled, distilled water, applied to a 0-9x12 cm column of CM Sephadex equilibrated with 0-05M-sodium phosphate buffer, pH 5-7, and eluted with a gradient of 50 ml 0-05M-sodium phosphate, pH 5-7, in the mixing chamber and 50 ml 0-1 M-sodium phosphate, pH 7-0, in the reservoir. Fractions (1 ml) were collected and assayed by their pressor effect on the rat blood pressure preparation (Plate). 0-1 ml of fractions containing AI or All were pooled, bioassayed against the respective AI or AH standard, and also tested by radio¬ immunoassay for All (Dusterdieck & McElwee, 1972). The total amounts of angio¬ tensin in each peak and the percentage recovery of angiotensin from the column could then be found.Complete separation of standard l-asp-5-ile AI and l-asp-5-ile AH was obtained (Plate). The mean recovery of standard AI in four experiments was 76 % and that of AH was 82%. The concentration of All found by radioimmunoassay of an All peak (77-4 ng/ml) agreed with that found by bioassay (60-80 ng/ml). Material from the AI fraction contained 300-400 ng angiotensin/ml by bioassay, but only 5-4 ng/ml