A novel tripeptidic renin inhibitor is described, SQ 32,970, that will potently inhibit endothia protease. This inhibitor can be coupled to Sepharose and will allow the affinity-purification of endothia protease in one step to greater than 95% purity as measured by SDS PAGE. The purified endothia protease cleaves the Lys-Pro-Ala-Glu-Phe-Nph-Arg-Leu substrate at the Phe-Nph bond with a Kcat/Km of 7445 (s-1 mM-1) at pH 3.1 and 4057 (s-1 mM-1) at pH 6.0. Affinity purified endothia protease can be crystallized in the pH range in which it is enzymatically active and can be inhibited by renin inhibitors.