The primary structure of the 70 kDa subunit of soluble bovine guanylate cyclase, which catalyzes the formation of cyclic GMP from GTP, has been determined. The alignment of six different clones out of two bovine libraries yielded a total of 3.1 kb with a coding region of 1857 bases. The open reading frame encodes a protein of 619 amino acids and a molecular mass of 70.5 kDa. Antibodies raised against a synthetic peptide, which corresponded to the C-terminus of the deduced sequence precipitated guanylate cyclase activity from guanylate cyclase-enriched preparations.