Activity coefficient values for NaCl and glycine are determined by studying their aqueous mixtures at different electrolyte and glycine concentrations. The mean ionic activity coefficient of NaCl was calculated from cell potential measurements: Na-ISE/NaCl (m(NaCl)); glycine (m(Gly))/Ag/AgCl. Measurements were made over the concentration range of (0.005 to 2) mol.kg(-1) glycine and of (0.05 to 2) mol.kg(-1) NaCl at temperatures (298.2, 308.2, and 318.2) K. The experimental data were correlated with the Roberts-Kirkwood and Scatchard-Prentiss virial expansion series. The activity coefficient of glycine is related to the mean ionic activity coefficient of the electrolyte through the cross-differential relation and evaluated by the standard deviation, sigma. The mean ionic activity coefficient of NaCl decreases as the molality of glycine in the solution increases. The presence of NaCl decreases the activity coefficient of glycine and consequently represents a salt-in effect for amino acid.