Here we describe a simple method to estimate the inner-sphere hydration state of the Mn(II) ion in coordination complexes and metalloproteins. The line width of bulk (H2O)-O-17 is measured in the presence and absence of Mn(11) as a function of temperature, and transverse O-17 relaxivities are calculated. It is demonstrated that the maximum O-17 relaxivity is directly proportional to the number of inner-sphere water ligands (q). Using a combination of literature data and experimental data for 12 Mn(II) complexes, we show that this method provides accurate estimates of q with an uncertainty of +/- 0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7 T and higher. The hydration number can be obtained for micromolar Mn(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn(II) binds to the multimetal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 x 10(8) s(-1) at 37 degrees C). The possibility of extending this technique to other metal ions such as Gd(III) is discussed.