A basic protein of apparent molecular weight 15 kD, designated bSVSP15, was purified from bovine seminal vesicle secretion to homogeneity, employing affinity absorption to calmodulin-Sepharose and reverse-phase HPLC. Immunoblotting identified bSVSP15 in bovine seminal plasma and seminal vesicle secretion, but it was not present either in extracts of bovine ampulla, epididymis, and testis or in serum or follicular fluid. When added to cAMP phosphodiesterase, bSVSP15 inhibited the activation of enzymatic activity by calmodulin in a reversible manner. Immunoscreening of a lambda gt11 expression cDNA library from bovine seminal vesicle tissue yielded two positive clones, pSVS4 and pSVS5, which were characterized by sequencing. Both sequences are identical, except for the 3' region. Because the derived amino acid sequence comprises, with an identity of 81%, the amino-terminal 21 residues of bSVSP15, cDNA clones pSVS4 and pSVS5 represent bSVSP15-specific mRNAs. The mature protein bSVSP15 contains 101 residues and is preceded by 25 residues of a signal sequence, characteristic for secretory proteins. Northern analysis identified two bSVSP15-specific mRNAs of 900 bp and 1200 bp, respectively. Sequence comparison yielded high homologies to human C-type natriuretic peptide. We conclude from this result that bSVSP15 is identical with the hitherto unknown bovine C-type natriuretic peptide.