A high molecular form of angiotensin-converting enzyme with mol mass about 600 kDa was found simultaneously with the well-known low molecular enzyme form of 190 kDa after fractionation of freshly prepared extracts from bovine kidney cortex and lung tissues by means of ammonium sulfate or gel filtration on Sephadex G-200. The rate of substrate hydrolysis was adequately Cl'-dependent for both these enzyme forms and specific inhibitors nonapeptide SQ 20881 and pentapeptide SQ 20475 inhibited similarly their activity. The enzyme high molecular form transformed into its low molecular derivative after storage, in freezing-thawing and ultrafiltration. Aprotinin, the inhibitor of serine proteinases, inhibited this kind of transformation. Stable form of the high molecular angiotensin-converting enzyme, which did not transform into its low molecular derivative, was obtained after treatment with agarose-immobilized aprotinin. Endogenous serine proteinases, which may regulate the angiotensin-converting activity in vivo, appears to be responsible for the enzyme transformation into its low molecular derivative.