The magnetic field dependence of water proton relaxation rates of diferric transferrin solutions has been analyzed over the 0.01-200-MHz range. There is evidence of both fast and slow exchanging protons. The former provide the shape of the proton relaxation rate profile. The effect of ZFS and its disappearance when the Zeeman energy dominates the ZFS provide an inflection in the nuclear relaxation dispersion profile around 10 MHz. The experimental profiles can be quantitatively interpreted by taking into account (i) a rhombic ZFS of the iron(III) ions, (ii) the change of the quantization axes along which the electron relaxation rates are defined on passing from the ZFS limit to the Zeeman limit, and (iii) the usual Zeeman limit field dependence of the electron relaxation rates.