The sodium concentration and the physicochemical state of sodium ions in biological polyelectrolytes may be an indirect measure of the macromolecular matrix integrity. Nuclear magnetic resonance (NMR) microscopy was used to measure the sodium concentration and to estimate the cartilage matrix fixed charge density (FCD) in control, acid-neutralized, and enzyme-digested bovine nasal cartilage (BNC), a model for human cartilage. The mean ion activity coefficient of sodium in cartilage was calculated 6 om the measured tissue sodium concentration as a function of external NaCl concentration and compared with the predictions of the Debye-Huckel and the modified Manning theories. Our results show that (1) the FCD in bovine nasal cartilage (at physiological NaCl concentrations) is in the range of 0.25-0.35 m, (2) the sodium content measured by NMR microscopy reflects indirectly the changes of FCD induced by changes in pH and enzymatic digestion, and (3) the measured activity coefficient for NaCl in the bovine nasal cartilage matrix is significantly lower than that of an ionic solution of the same salt concentration. An extension of the model developed by Manning for polyelectrolytes is in good qualitative agreement with our experimental results which showed a decrease in the mean activity coefficient with decreased reservoir concentration.