Different forms of renin have been purified from bull kidney by combined gel filtration, affinity chromatography and ion-exchange chromatography. The specific activity of the enzyme was determined by a biochemical method of synthetic substrate and by radioimmunoassay on both synthetic and natural substrates; molecular characterization was carried out by molecular weight determinations, polyacrylamide gel electrophoresis, isoelectric focusing, amino acid analysis and optical rotatory dispersion. Three forms (renin C, D, E) are distinct on the basis of amino acid composition and chromatographic behavior, while possessing the same molecular weight, and displaying only minor differences in specific activity, alpha-helix content and isoelectric point; the occurrence of a group of renin isoenzymes may be suggested. Another form (A) has a lower specific activity and a higher molecular weight (57 000) compared with C, D and E and further differs markedly in chromatographic behavior, amino acid composition, alpha-helix content and isoelectric point, as well as in substrate specificity; it may be regarded as a pseudorenin. The fifth form (B) possesses the highest specific activity and does not correspond to a single molecular form; the presence of two components of different molecular weight (27 000 and 46 000 respectively) has been established both by polyacrylamide gel electrophoresis and isoelectric focusing.