Angiotensin converting enzyme (EC 3.4.15.1) from bovine seminal plasma was shown to exist in two distinct forms. A lower molecular weight form of the enzyme having higher activity was purified to homogeneity. Final recovery of the enzyme was 18.37%. The apparent molecular weight of the enzyme was estimated to be 1.99 x 10(5) by gel filtration. A value of 1.8 x 10(5) was obtained for the reduced and denatured enzyme by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Stokes' radius, diffusion coefficient and intrinsic viscosity were determined to be 51.89 A degree, 4.2 x 10(-7) cm2/sec and 4.42 cc/g, respectively. Chloride ions were required for the enzyme activity. Studies with EDTA suggest that metal ions which are tightly bound, are required for its activity. Enzyme was inhibited by some heavy metal ions and required disulfide linkages at its active site. Trypsin treatment of the urea denatured enzyme produced a catalytically active Mr 32,000 daltons fragment.