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A general procedure for investigating cross relaxation is presented. Experimental evidence derived from proton NMR studies of collagen and muscle clearly shows that cross relaxation between the bulk of the water protons and the bulk of macromolecular protons significantly perturbs water spin-lattice relaxation. These findings have serious implications for interpretations of the molecular dynamics of water molecules in hydrated biological systems. The presence of cross relaxation is direct proof that a fraction of the water protons exchange at a rate slow compared with the Larmor frequency.