A new stereoisomer of matrine was isolated from “Ku-Shen,” dry roots of Sophora fiavescens, and named (+)-isomatrine. Epimerization of this compound into (+)-matrine (I) demostrated its flamework. X-Ray analysis of (+)-isomatrine hydrobromide was performed and its structure, [5R,6R,7S,11R]-17-oxomatridine (VI), was elucidated. The conformations of A, B, C and D ring in VI are chair, boat, boat and half-chair, respectively. © 1975, The Pharmaceutical Society of Japan. All rights reserved.