Electron-nuclear double resonance (ENDOR) spectroscopy has been used to study ligand and copper hyperfine interactions in Cu(II) complexes of human transferrin. A nearly isotropic superhyperfine interaction of the Cu(II) spin with a single 14N nucleus was identified, and the principal values of its tensor were estimated. All principal values of the copper hyperfine tensor were also directly measured for the first time. Resonances from at least two exchangeable protons were observed, but their origin could not be ascertained. At physiological pH, and in the presence of bicarbonate, ENDOR spectra of the two metal-binding sites were virtually indistinguishable.