To prove the applicability of the Fitter equations, modified by us after taking into account the dependence of the solution concentration on the mixing parameters related with the binary solute-solute interactions, we are presenting the analysis of accurate experimental activity coefficient values here determined for the mixed system NaCl + beta-alanine + H2O up to medium concentrations of both NaCl and the aminoacid. These activity coefficient values were obtained from the emfs of the galvanic cell : Na-glass \ beta-Alanine (m(A)), NaCl (m(B) = I), AgCl \ Ag. The fitting was done by considering only the binary interaction mixing parameters, i.e. by neglecting the ternary ones (ion-zwitterion-zwitterion and ion-ion-zwitterion). The standard deviation thus found for the fitting (+/-0.16 mV, i.e. +/-0.003 in ln gamma(+/-)) was of the same order as the accepted experimental uncertainty. Activity coefficients for beta-alanine in these aqueous solutions, calculated from the use of these modified Fitter equations, are also presented.