Small colloids in a uni-univalent aqueous electrolyte are modeled by a four-component solvent primitive model. The symmetric Poisson-Boltzmann theory is used to analyze the system and to calculate the second virial coefficient B-2 of very dilute particle suspensions for varying solute and solvent concentrations. The results are compared with some experimental results on B-2 of the silicotungstate anion suspension in an alkali halide solution.