2, 3-Oxidosqualene : β-amyrin cyclase (E.C.5.4.99) was purified from pea seedlings in eight steps as a soluble and homogeneous enzyme. The purified enzyme showed a single band in SDS-polyacrylamide gel electrophoresis (SDS-PAGE) with a molecular weight of 35 K Da and had a Km value of 50 μM. It was demonstrate that β-amyrin cyclase had a different molecular weight from that of cycloartenol cyclase, and these two enzymes were responsible for the dramatic alteration in triterpenoid and steroid biosynthesis during germination.