Bactenecins are highly cationic polypeptides of the large granules of bovine neutrophils, exerting in vitro a potent antimicrobial activity. Two bactenecins, with an approximate molecular weight of 7000 and 5000, called Bac7 and Bac5, are characterized by a high content of proline (greater than 45%) and arginine (greater than 23%) residues. Their complete amino acid sequences were determined by automated Edman degradation combined, in the case of Bac5, with plasma desorption mass spectrometry. Bac7 comprises 59 residues and includes three tandem repeats of a tetradecamer characterized by several Pro-Arg-Pro triplets spaced by single hydrophobic amino acids. Resolution of the primary structure of Bac5 required fragmentation with N-bromosuccinimide as well as digestion of the obtained C-terminal fragment with carboxypeptidases P and Y directly in the mass spectrometer. Bac5 comprises 42 amino acid residues with a repeated motif of Arg-Pro-Pro triplets also alternating with single apolar residues.