The solubility of barium polystyrenesulfonate in water and in aqueous solutions of Lithium, sodium, and potassium chloride at various concentrations was determined at 25 degrees C. From these data the relative mean activity coefficients of the polyelectrolyte were calculated. A strong decrease of these coefficients with increasing concentration has been found. The experimental results have been compared with theoretical predictions obtained in the Poisson-Boltzmann approximation for the classical cell model of polyelectrolyte solutions. An attempt has also been made to include the short-range forces, represented by a square-well potential, into the equations. Ld this way a better agreement between theory and experiment has been obtained.