Oxygen-17 NMR studies of caseins isolated from fresh milk of several species were carried out in concentrated electrolyte solutions as a function of both protein and electrolyte concentration. Molecular dynamics simulations of ion and water binding to caseins are compared with the experimental observations by O-17 NMR of caseins in solutions with ions. Protein activities are also determined by utilizing Wyman's theory of linked functions with a detailed model of ion binding to casein.