Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation measurements at 4.7 T were used to study the hydration properties of bovine and caprine casein solutions with and without NaCl. Measurements were carried out at 21 °C and pD 6.95. Nonlinear protein concentration dependences were observed for the oxygen-17 NMR transverse relaxation rates, but the fitting of the data did not require any higher order virial coefficients; only So was needed. This indicates that long-range, charge-charge repulsive interactions dominate entirely the protein activity in solution. Estimates of the bovine casein average “net” charge were obtained from the virial coefficient (Bo) and the partial specific volume; these are in accord with published amino acid sequence data. For bovine casein, the αS1- and β-components occur in nearly equal amounts, whereas in caprine casein, β-casein is the predominant species and αS1-casein varies from high to low values, suggesting altered protein-protein interactions. Hydration levels of caprine casein high in the αS1-casein component when compared with those of bovine casein (intermediate hydration) and the low αS1- caprine casein (low hydration) are interpreted in terms of “trapped water” and “preferential interactions” with water on the basis of quantitative differences in casein monomer contents. © 1995, American Chemical Society. All rights reserved.