The solution structure is reported for bovine neutrophil beta-defensin-12 (BNBD-12), a member of the beta-defensin family of antimicrobial peptides. Structural constraints in the form of proton-proton distances, dihedral angles, and hydrogen bond constraints were derived from two-dimensional, homonuclear magnetic resonance spectroscopy experiments. The three-dimensional structure of BNBD-12 was calculated using distance geometry and restrained molecular dynamics. An ensemble of structures with low NOE constraint violation energies revealed a precisely defined triple-stranded, antiparallel beta-sheet as the structural core of the peptide. The N-terminal beta-strand and three locally well-defined tight turns form a hydrophobic face. Conserved isoleucine and glycine residues form a beta-bulge structure which initiates a beta-hairpin secondary structure motif composed of the second and C-terminal beta-strands. The beta-hairpin contains numerous charged residues and forms the cationic face of BNBD-12. The N-terminal residues were found to be disordered, due to an absence of tertiary NOEs. The triple-stranded beta-sheet, the beta-bulge preceding the hairpin, and the cationic/hydrophobic amphiphilic character are definitive features of all defensin structures determined to date. Further, we predict that the tracheal antimicrobial peptide (TAP) and the recently described gallinacins will have tertiary structures similar to that of BNBD-12.