The 81Br nuclear magnetic resonance line width in aqueous bromide solutions is broadened upon the addition of various proteins. Comparison of metallo-enzymes with metal-free proteins gives evidence for bromide ion binding independent of metallic sites. The effect of temperature and pH on the 81Br resonance has been studied. Some measurements on 85Rb, 79Br and 35C1 resonances are included for comparison. © 1969.