The intrinsic interactive forces between two free colloidal particles suspended in aqueous solutions have been measured as a function of the separation by applying the optical trapping technique to a monodispersed polystyrene latex of a mean diameter 2.13 mu cm. For this purpose, specific technology for the precise measurement of the surface-to-surface separation and force strength has been developed. The results were in excellent agreement with the prediction of the DLVO theory in a variety of electrolyte concentrations.