AbstractMeasurements of the spin‐lattice relaxation rates of water protons made over a wide frequency range have demonstrated that the effects of paramagnetic relaxation agents may be considerably enhanced when the paramagnetic center is incorporated into a compact structure macromolecule such as a protein that is rotationally constrained. The immobilization of the macromolecule profoundly changes the nature of the magnetic field dependence of the relaxation rate for both the diamagnetic and the paramagnetic samples. The immobilization also amplifies the effect of the paramagnetic center as a water‐proton relaxation agent. The direct exchange of labile water molecules or protons is not a requirement for the high efficiency of this class of magnetic relaxation agents. © 1992 Academic Press, Inc.