Reduction and carboxymethylation of mouse submaxillary gland renin produced two polypeptide chains which were readily separated by gel filtration or sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weights of the two chains, termed heavy chain and light chain, were determined to be 30,000 and 5,458, respectively. Carboxymethylation of renin with radiolabeled iodoacetic acid followed by chain separation after reductive cleavage of disulfide bridges revealed the presence of two free cysteine residues in the heavy chain. Based on the finding of one half-cystine in the light chain and three half-cystine residues in the heavy chain. It was concluded that the heavy and light chains are linked by one disulfide bridge and that the heavy chain contains an intrachain disulfide bridge. The complete 48-amino acid residue sequence of the light chain was determined using peptide fragments obtained by cyanogen bromide cleavage and digestion with Staphylococcus aureus protease. The sequence showed 46% homology with the carboxyl-terminal region of the porcine pepsin sequence.