CBZ amino acid p-nitrophenyl esters are excellent substrates for (a) a commercially available renin preparation, (b) a purified hog renin sample and (c) the World Health Organization standardized human renin. The Michaelis-Menten parameters for the commercial hog renin are K(m) = 4.1 X 10^-5 mol/1 and V (max) — 2. 1 min^-1 • mg-1 at pH 7.40 and 25 °C in tris buffer containing 100 ml/I ethanol. Column chromatography on DEAE cellulose using a descending acetate pH gradient separated esterase activity as measured by nitrophenyl ester hydrolysis and renin activity as measured by radioimmunoassay. The esterase activity was not due to angiotensinase contamination. Esterase activity was almost completely abolished by treatment with phenylmethylsulfonyl fluoride; renin activity was only slightly affected. It was concluded that the esterase activity was not due to renin.