[en] The procedure ensures that the free or bound activity content in radioimmunoassays can be determined selectively without their previous separation, if the radionuclide ¹²⁵I is being used for. It is also possible to determine the activity share existing at the threshold region between a solution and a solid or colloidal phase. A solid or colloidal layer with scintillation properties and binding capacities for free or bound activities has been brought into the solution. The activity at the boundary layer has been measured. The measured values can be assigned to the corresponding concentrations by means of standards